This chapter is very important from examination point of view. Here we have discussed the concepts like ” light, sources of light, reflection of light, refraction of light, spherical mirrors and their uses, spherical lenses and their uses, image formation by mirrors and lenses, power of a lens and linear magnification”. These concepts are very important from the examination point of view and you should go through these concepts very well so as to make your foundations stronger.
Light Reflection and Refraction Class 10 Science Questions and Answers
In the forthcoming section, we have provided you Light reflection and refraction class 10 notes in the form of questions and answers. These notes have been prepared by a team of experts who have an experience of more than 10 years of teaching at secondary level. We are sure that you will find these notes very useful.
Light
Light is a form of energy which causes sensation of sight. Broadly speaking, light refers to all the electromagnetic waves, but from the optics point of view, light means the visible light, that our eyes are sensitive to.
Ray of light:
Straight line drawn in the direction of propagation of light is termed as ray of light.
Types of sources of light:
The natural source of light is sun. Other man-made (artificial sources) of light are electric bulb, oil lamp, fluorescent tube, sodium lamp. mercury iamp etc. The sources emitting light of their own are called self-luminous. For example the sun. glow worm (i.e., Jugnu) etc.
The sources which do not possess their own light but give light called reflect light of other luminous sources are called non-luminous sources, e.g. moon, earth, paper etc.
Speed of light:
Light travels with different speeds in different media.
a) Speed of light in vacuum = 3 x 108m/s
b) Speed of light in air is almost same as in vacuum, (3×108m/s)
c) Speed of light in water = 2.25 x 108m/s
d) Speed of light in glass = 1.80 x 108m/s. Incident ray: It is defined as light traveling in first medium.
Light has the maximum speed in vacuum. Nothing can exceed this speed limit.
Point of incidence:
It is a point at which incident ray strikes the reflecting surface.
Angle of incidence:
The angle which the incident ray makes with the normal at the point of incidence is called angle of incidence. It is represented by ∠i.
Angle of reflection:
The angle which the reflected ray makes with the normal at the point of incidence is called angle of reflection. It is represented by ∠r.
Medium of propagation:
It may be defined as the path or way through which light passes.
What are spherical mirrors? Give sign conventions for spherical mirrors.
Spherical mirror: Spherical mirror is a part of hollow sphere with one side highly polished to reflect almost all the light falling on it. Spherical mirrors are of two types: ) Concave spherical mirror (B) Convex spherical mirror
- Concave spherical mirror: A concave mirror is that spherical mirror whose outer bulged surface is polished and reflection of light takes place at the concave surface (bent-in surface). The inner shining surface of a spoon is an example of concave mirror.
- Convex spherical mirror: A convex mirror is that spherical mirror whose inner surface is polished and the reflection of light takes place at convex surface (bulged out surface).
Definitions related with spherical mirrors?
A) Pole: The middle point or centre of the spherical mirror is called pole mirror. It is represented by letter “P”.
B) Centre of curvature: It is the centre of hollow sphere of which the mirror is a part. It is represented by “C”. The centre of curvature of a convex mirror is behind it and that concave mirror is infront of it.
C) Principal axis: The imaginary line passing through centre of curvature and pole of a spherical mirror is called principal axis.
D) Principal focus: The principal focus is a point where a beam of light initially parallel to principal axis appears to converge (meet) after reflection or appears to diverge after reflection from the mirror. It is denoted by “F”
Principal focus of a concave mirror: The principal focus of a concave mirror is a point where a beam of light initially parallel to the principal axis, actually meet after reflection from the mirror.
Principal focus of a convex mirror: The principal focus of a convex mirror is a point where a beam of light initially parallel to the principal axis, appears to diverge after reflection from the mirror.
E) Focal length: It is the distance between the pole and the principal focus of a mirror. It is represented by “f”.
F) Aperture: It is the effective diameter of reflecting area (surface) of the mirror. Aperture represents the size of the mirror.
G) Radius of curvature: Radius of curvature of a spherical mirror is the radius of the sphere of which mirror is a part. It is represented by the letter “R”.
H) Principal section: A section of a spherical mirror cut by a plane passing through the pole and centre of curvature of the mirror is called principal section of the mirror.
Relation between radius of curvature and focal length
The focal length of a spherical mirror is equal to half the radius of curvature of the mirror.
where R=radius of curvature and f= focal length. This formula is valid for both concave and convex mirrors.
Sign convention used in spherical mirrors:
The new Cartesian sign conventions used for measuring various distances in the ray diagrams of the spherical mirrors (convex or concave) are summarized as follows:
- All distances are measured from the pole of the spherical mirror pole is taken as origin.
- The distances measured in the direction of incident ray are taken as positive while those measured in the direction opposite to the incident light are taken as negative.
- Height of object or image measured in upward direction and perpendicular to the principal axis is taken as positive.
- Heights of object/image measured in downward direction and perpendicular to principal axis is taken as negative.
- The object is taken on the left hand side of the mirror.
- Distance in real image is negative while in virtual image it is positive.
- The principal axis of the mirror is taken along X-axis.
Define Principal focus and focal length of a concave mirror?
Principal focus of a concave mirror: The principal focus of a concave mirror is a point where a beam of light initially parallel to the principal axis, actually meets after reflection from the mirror.
Focal length: It is the distance between the pole of a spherical mirror and its principal focus. It is represented by “F”.
Discuss the image formation in case of a concave mirror with the help of ray diagram?
The type of image formed by a concave mirror depends upon the position of the object infront of the mirror. The following six cases arise:
Case1: When the object is at infinity: When an object is at very large distance from a concave mirror, it is said to be at infinity. Two rays of light AD and E’E coming parallel to the principal axis, get reflected along DF’ and EF’. The image is formed at the focus (F). The image is real, inverted and highly diminished.
Case 2: When the object is placed beyond the centre of curvature: A ray of light AD coming parallel to the principal axis gets reflected along DF. Another ray of light AE passing through centre of curvature is reflected along EC. (this ray retraces its path) The image is formed between centre of curvature (C) and focus (F). The image is real. inverted and diminished.
Case 3: When the object is placed at centre of curvature: A ray of light AD parallel to the principal axis is reflected along DA’. Another ray of light AE passing through the focus is reflected along EA’ parallel to the principal axis. The image is formed at centre of curvature (C) and is real, inverted and of the same size as that of the object.
Case 4: Object between focus and centre of curvature: When the object AB is placed between (F) and (C). A ray of light AD parallel to principal axis passes through focus (F) after reflection at D. Another ray of light AE passes through centre of curvature (C) after reflection at E (it retraces its path ). The two rays actually meet at A’. Thus forming the image A’B’. It is real, inverted and magnified image and is formed beyond the centre of curvature.
Case 5: Object placed at focus (F): If the object is placed at F focus, all the rays starting from the object become parallel to the principal axis after reflection from mirror and therefore meet at infinity. Thus image formed is at infinity. The image formed is real, inverted and magnified, this image can’t be obtained on screen.
Case 6: Object between pole and focus: A ray of light AD parallel to principal axis passes through focus (F), after reflection at D. Another ray of light AE passes through centre of curvature C after reflection at E (this ray retraces its path). The two reflected rays DF and EAC diverge and cannot meet actually.
However, when these two reflected rays are produced backwards, they appear to come from a common point A’ behind the mirror. Thus A’B’ is the image of the object, which is virtual, erect, magnified and lies behind the mirror. As the image is virtual, it cannot be obtained on screen.
The above results have been summarised in the table given below:
| Position Of Object | Position Of Image | Size of Image | Nature of Image |
|---|---|---|---|
| At Infinity | At Focus F | Highly dimnished | Real and Inverted |
| Beyond C | Between C and F | Dimnished | Real and Inverted |
| At C | At C | Equal to size of object | Real and Inverted |
| Between C and F | Beyond C | Enlarged | Real and Inverted |
| At focus F | At Infinity | Highly enlarged | Real and Inverted |
| Between F and P | Behind the mirror | Enlarged | Virtual and erect |
Describe the rules for tracing the Images formed by a Convex Mirror?
The rules for tracing the images formed by a convex mirror are given below:
Rule 1): A ray of light falling on the mirror in a direction parallel to the principal axis of a convex mirror appears to be coming from its focus on reflection from the mirror as shown in figure.
Rule2): A ray of light directed towards centre of curvature of a convex mirror is reflected back along the same path i.e. such a ray of light retraces its path on reflection as shown in figure.
Rule3): A ray of light directed towards focus of a convex mirror becomes parallel to the principal axis of mirror after reflection. This rule is just the reverse of Rule1.
Rule4): A ray of light incident obliquely towards the pole P of a convex mirror is reflected obliquely, such that the incident ray and the reflected ray make equal angles with the principal axis.
Discuss the image formation by a convex mirror with the help of a ray diagram?
The image formed by a convex mirror is always behind the mirror. The image formed is virtual, erect and smaller in size, whenever the distance of the object is changed from convex mirror, then only the position and the size of the image changes. There are two main positions of object in case of a convex mirror from the point of view of position and size of image:
(i) At infinity and (ii) Anywhere between pole (P) and infinity.
Case 1: When the object is placed at infinity: A ray of light AP inclined to the principal axis is reflected at P along PG. Another ray of light AD is reflected at D. along DE. The two reflected rays PG and DE, when produced back intersect at point A. Thus A’ B’ image is formed at focus (1) behind the mirror, the image formed is virtual, erect and highly diminished in size.
Case 2: When the object is placed between infinity and the pole of the mirror: A ray of light AD parallel to principal axis is reflected at D, along DE. Another ray of light AG retraces its pass on refiection at G. the two reflected rays DE and GA when produced back intersect at A. Thus A’B’ is the image formed, which is virtual, erect, and diminished in size and lies behind the mirror between P and F.
Define linear magnification?
The linear magnification produced by a concave mirror is defined as the ratio of height the image (hi) to the height of the object (ho). It is represented by “m”.
Linear magnification (m) = height of image (hi) / height of object (ho)
Linear magnification of a spherical mirror is also defined as the ratio of the distance of the image (v) to the distance of the object (u) from the pole of the mirror.
m= – {distance of image (v) / distance of object (u)}
That is linear magnification m = – v/u
Linear magnification has no units, as it is the ratio of two similar quantities.
| Value of “m” | What it menas |
|---|---|
| m > 1 | It means the size of the image is greater than the size of object i.e. then hi>ho. |
| m < 1 | It means the size of the image is smaller than the size of object i.e. then hi<ho. |
| m = 1 | It means the size of the image is equal to the size of object i.e. then hi=ho. |
| m = +ive | It means the image formed is virtual and erect. In this case, Image formed always lies above the principal axis. |
| m = -ive | it means the image formed is real and inverted. In this case, Image formed always lies below the principal axis. |
What are the uses of spherical mirrors?
(A) Uses of concave spherical mirrors:
- They are used as a reflector in torches, search lights, headlights of motor vehicles etc. to get powerful parallel beams of light.
- Concave mirror is used as a shaving mirror as it can form erect and magnified image of the face.
- They are used by doctors to concentrate light on body parts like ieeth. cars and eyes. which are to be examined.
- Large concave mirrors are used to concentrate sunlight to produce heat in sciar cookers, solar furnaces etc.
- Concave mirrors of large diameters are used as objectives in reflecting telescope.
(B) Uses of convex spherical mirrors:
- A convex mirror is used as a reflector in street lamps. As a result, light from the lamp diverges over a large area.
- A convex mirror is used by drivers of automobiles like cars. buses and trucks as a rear view mirror.
What is Refraction?
The phenomenon of change is the direction of light when it goes from one medium to another is called refraction. In other words refraction is the phenomenon in which bending of light takes place when it passes from one medium to another medium. The refraction takes place at the boundary of two media. The basic cause of refraction is change in the velocity of light in going from one medium to another medium.
In given figure the incident ray traveling in air medium along “AO” and when it enters glass medium at point “O”, it bends and goes along “OB”.
In the given figure “AO” is incident ray, “OB” is refracted ray, ∠BON is angle of refraction and ∠OAN is angle of incidence, ON is the normal.
It has been found that when a-ray of light goes from an optically denser medium to an optically rarer medium, it bends away from the normal at the point of incidence. And when it goes from a optically rarer medium to an optically denser medium it bends towards normal at the point of incidence.
Laws of refraction
Ist Law of Refraction :-It states that the incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane. In given figure, the incident ray “AO” the refracted ray “OB” and the normal “ON” all lie in the same plane. i.e. plane of paper
2nd Law of Refraction :- This law gives the relationship between the angle of incidence and angle of refraction. The law was given by Snell in 1621, so the 2nd law is also called as Snell’s law of refraction. It states that the ratio of sin of angle of incidence to the sin of angle of refraction is constant for a given medium.
i.e. (sin i) / (sin r) = constant
This constant is called as refractive index and is denoted by mew (μ). Thus refractive index μ = (sin i) / (sin r)
3rd Law of Refraction :- It states that whenever light goes from one medium to another, the frequency of light and phase of light do not change. However, the velocity of light and the wavelength of light changes.
Refractive index
The refractive index of a medium is defined as the ratio of speed of light in vacuum to the speed of light in the given medium. It is represented by “μ“.
Refractive index of a medium (μ) = speed of light in vacuum / speed of light in given medium
As the speed of light in air is almost equal to speed of light in vacuum. Therefore,
μ = speed of light in air (C) / Speed of light in given medium (V)
e.g, refractive index of glass μg = speed of light in air / speed of light in glass
Refractive index has no unit (as it is ratio of two velocities).
Relative Refractive Index
When light passes from medium1 to another medium 2. The refractive index of medium 2 with respect to medium 1 is written as 1μ2 ‘ and is called relative refractive index. If μ1 is the refractive index of medium 1 and μ2 that of medium 2 Then, 1μ2 = μ2 / μ1——————-(1)
If v1 is the speed of light in medium 1 and v2 in medium 2,
Then μ1 = C/v1 and μ2 = C/v2 ——— (2)
Substitute (2) in (1), we get, 1μ2 = v1/v2
Similarly, 2μ1 = v2/v1
Thus refractive index of medium 2 with respect to medium 1 is reciprocal of refractive index of medium 1 with respect to medium 2.
What is Lens?
A lens is a piece of transparent material (usually glass) with atleast one curved surface having some magnifying power.
Types Of Lenses
The lenses can be broadly classifies into two types. They are:
1. Convex lens: The lens which is thicker at centre and thinner at edges is called a convex lens.
2. Concave lens: The lens which is thinner at centre and thicker at edges is called a concave lens.
Define the principal focus & focal length of a convex lens?
A convex lens has two surfaces & hence it has two focal point or principal foci.
First principal focus of a convex lens is the position of point object on the principal axis of the lens, for which the image formed by the lens is at infinity. It is represented by F1.
First focal length: The distance of first principal focus of the lens from optical centre C of the lens is called first focal length of convex lens. In above figure, f1 = CF1 It is represented by f1.
Second Principal Focus of a convex lens is the position of an image point on the principal axis of the lens, when the point object is situated at infinity. It is denoted by F2. It is a real point.
Second focal length: The distance of 2nd principal focus of the lens from the optical centre “C” of the lens is called second focal length of convex lens. It is represented by f2. Thus in given figure f2 = CF2.
Define principal focus & focal length of a concave lens?
A concave lens has two surfaces & hence it has also two focal points or two principal foci.
(1) First principal focus of a concave lens is the virtual position of the point object on the principal axis of the lens, for which the image formed by concave lens is at infinity. It is represented by f1.
First principal focal length of concave lens: The distance of first principal focus of the lens from optical centre ‘C’ of the lens is called first principal focal length of concave lens. It is represented by fi Thus in figure fı = CF1.
(2) The Second principal focus of a concave lens is the position of the image point on the principal axis of lens, when the point object is situated at infinity. It is represented by F2, It is virtual point.
Secord Principal focal length of concave lens: The distance of second principal focus of the lens from the optical centre of the lens is called 2nd principal focal length of concave lens. It is represented by f2. Thus in given figure f2 = CF2
What are the rules used for obtaining image formed by convex lenses?
When an object is placed in front of a convex lens, an image is formed, at that point where at least two refracted rays meet (or appear to meet).
Rule 1:- A ray of light which is parallel to the principal axis of a convex lens, passes through its (second) principal focus after refraction through the lens as shown in fig
Rule 2:- A ray of light passing through the optical centre of a convex lens passes straight after refraction through the lens as shown in figure
Rule 3 :- A ray of light passing through the (first) principal focus of a convex lens becomes parallel to its principal axis after refraction through the lens as shown in figure.
Discuss the image formation by convex lens with the help of ray diagram?
The type of image formed by a convex lens depends on the positions of the object in front of lens. Following six cases arise.
Case 1:- When the object is placed at infinity: Two parallel rays of light AC and AD are inclined to the principal axis of the lens. The ray AC passes undeviated through optical centre and ray AD converges on refraction through the contes lens. The two refracted rays actually meet at A.
The image is formed at the second principal focus (F2) of the convex lens. The image is real inverted & highly diminished as shown in figure.
Case II:- When the object is placed beyond 2F: A ray of light starting from A and incident on the lens along AD in a direction parallel to the principal axis of the lens, on refraction passes through second principal focus F2 of the lens. Another ray of light starting from A and incident on the lens along AC. goes undeviated through the optical centre of the lens. The two refracted rays meet actually at A.
The image formed is between F2 & 2F2. The image is real inverted & diminished as shown in figure
Case III:- When the object is placed at 2F, (F is first principal focus):
A ray of light starting from A and incident on the lens along AD in a direction parallel principal axis of the lens, on refraction, passes through second principal focus Fy of the Another ray of light starting from A and incident on the lens along AC, passes undeviated through the optical centre. The two refracted rays meet actually at A’.
The image is formed at 2F2. The image is real, inverted & same in size as shown in B B F2 2F2 2F1 F1
Case IV: When the object is placed between F1 & 2F1: A ray of light starting from A and incident on the lens along AD in a direction parallel to the principal axis of the lens, on refraction, passes through second principal focus F2 of the lens. Another ray of light, starting from A and incident on the lens along AC, passes undeviated through the optical centre C of the lens. The two refracted rays meet actually at a point A’.
The image is formed beyond 2F2 on the other side of convex lens. The image is real, inverted & enlarged as shown in figure.
Case V: When the object is placed at F1: A ray of light starting from A and incident on the lens along AD in a direction parallel to the principal axis of the lens, on refraction, passes through second principal focus F2 of the lens. Another ray of light, starting from A and incident on the lens along AC, passes undeviated through optical centre C of the lens. The two refracted rays emerge from the lens in a direction parallel to each other, as shown in figure. Those rays would meet at very large distance from the lens, say at infinity. The image is formed at infinity. The image is real, inverted & highly magnified as shown in figure.
Case VI :- When the object is placed between F1 & ‘C’: A ray of light starting from A and incident on the lens along AD, in a direction parallel to the principal axis of the lens, on refraction, passes through second principal focus F2 of the lens. Another ray of light starting from A and incident on the lens along AC. passes undeviated through optical centre C of the lens. The two refracted rays from the lens are diverging and would not meet on the right side of the lens. However, when we produce the two refracted rays in the backward direction, they appear to come from the point A’.
The image formed is beyond f, on the same side of the lens the image formed is virtual, erect & enlarged as shown in figure.
What are the rules used for obtaining image formed by concave lens?
When an object is placed infront of a concave lens, an image is formed. The image is formed at that point where at least two refracted rays meet (or appear to meet).
Rule 1: A ray of light which is parallel to the principal axis of a concave lens, appears to be coming from its focus after refraction through the lens. as shown in fig
Rule 2: A ray of light passing through the optical centre of a concave lens goes straight after passing through the lens. as shown in fig
Rule3: A ray of light appearing to meet at the principal Focus of a concave lens after refraction will emerge parallel to the principal axis of the lens. This is shown in the fig.
Discuss the image formation by concave lens with the help of ray diagram?
The image is formed at a point, where the two refracted rays appear to meet. For all positions of the object, the image formed by a concave lens is virtual, erect and diminished in size. The exact position and size of the image would depend upon the position of the object. Two cases arise:
Case 1: When the object lies between optical centre and infinity: A ray of light AD starting from the top point A of the object is falling on the concave lens in a direction parallel to principal axis of the lens. This ray diverges after refraction along DE and on producing back; it appears to come from second principal focus F2 of the lens. Another ray of light AC starting from the same point A on the object, passes undeviated through the optical centre C, along ACG. The two refracted rays intersect at A’. Therefore, A’ is virtual image of the point A on the object.
When an object is held anywhere between optical centre C of concave lens and infinity, the image formed is: (i) Between optical centre C and second principal focus F2; on the same side of the lens and the image is, (ii) Virtual and erect and (iii) Smaller in size than the object.
Case 2: When the object is at infinity: When the object is at infinity point, image is formed at the second principal focus on the same side of the lens. This is shown in figure (b). The image is virtual, erect and highly diminished to almost point size.
What do you understand by linear Magnification produced by lenses?
The linear magnification produced by lenses is the ratio of the size of the image to the size of the object. It is represented by m.
Thus linear magnification = size of image / size of object.
If hi is the size of image and ho is the size of the object, then magnification (m) = hi/ho.
The linear magnification produced by a lens in terms of the image distance and object distance is equal to the ratio of image distance to the object distance.
Thus magnification = image distance / object distance.
If “I” is the image distance and “O” is the object distance. Then m=I/O
A concave lens forms an image which is always smaller than the object. Therefore linear magnification of a concave lens is always less than one (1).
In case of a convex lens:
(i) When the size of an image is equal to the size of the object i.e. hi=ho.
Therefore m=hi/ho = 1/1 = 1. thus the magnification is equal to one .
(ii) When the size of the image is greater than the size of the object,
i.e. hi is greater than ho as m = hi/ho > 1
then m is greater than one .Thus linear magnification is greater than one
(iii) when the image is smaller than object i.e. hi is less than ho. as m=hi/ho, then “m” is less than one.
Thus linear magnification is less than one.
Define Power of a lens?
The power of a lens is defined as the ability of lens to converge the rays of light falling on it. A convex lens converges the rays of light falling on it, power of convex lens is said to be positive.
A concave lens diverges the rays of light falling on it. Therefore power of a concave lens is said to be negative.
The power of a lens depends on its focal length. Mathematically, the power of lens is given by reciprocal of focal length of the lens i.e., Power of lens P= 1/Focal Length of lens (f).
Where ‘f is in metres. However if f is in cm then power of a lens is expressed as P = 100/f.
Clearly, smaller the focal length of a lens, greater is its power & Vice-versa.
For a convex lens, f is positive, therefore, ‘P’ is positive i.e., power of convex lens is positive.
For concave lens f is negative. So P is negative i.e. power of concave lens is negative.
The S.I unit of power of lens is dioptre, represented by “D”.
When f= 1m, p=1/1=1 dioptre.
Thus one dioptre is the power of a lens of focal length one metre.
The power of lens in diopters is called the number of lens e.g., a convex lens of focal length 50cm (f = 0.5m) has power = 1/0.5m = 2D. Its number is said to be +2.
Similarly for a concave lens of focal length 10cm (f = 0.1m) has power p = 1/0.1 m = 10D. Its number is said to be 10.
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So these were Light Reflection and Refraction Class 10 Notes. We are sure that you will find these very useful for your exam preparation.