CBSE Class 10 Physics Reflection and Refraction of Light Worksheet Set C

Light Reflection and Refraction Notes Class 10 Science

Light travels in a straight line- Speed of light in vacuum /air = 3 × 10 8 m/s.

Reflection- Bouncing back of light when it strikes on a polished surface like mirror.

Laws of reflection-
(i) Angle of incidence= Angle of reflection.
(ii) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

Concave mirror- Concave mirror is a part of a hollow sphere whose outer part is silvered and the inner part is reflecting surface.

Convex mirror- It is a part of a hollow sphere whose outer part is reflecting surface and inner part is silvered.

Centre of curvature- Centre of a hollow sphere of which the spherical mirror forms a part is called centre of curvature. Denoted by C.

Radius of curvature- Radius of a hollow sphere of which the spherical mirror forms a part is called radius of curvature. Denoted by R.

Pole- The mid point of a spherical mirror is called Pole. Denoted by P.

Aperture- The diameter of the part of a spherical mirror exposed to the incident light is called aperture of mirror.

Principal axis- A line joining the centre of curvature and pole of a spherical mirror and extended on either side is called Principal axis of spherical mirror.

Principal focus- A point on the principal axis of a spherical mirror where the rays of light parallel to the principal axis meet or appear to meet after reflection from spherical mirror is called Principal focus. Denoted by F.

Focal length- Distance between the pole (P) and the principal focus (F) of a spherical mirror is called focal length of spherical mirror.

f = R/2 , where R is the radius of curvature.
Focal length and radius of curvature of a concave mirror are negative.
Focal length and radius of curvature of a convex mirror are positive.
Radius of curvature of plane mirror= ∞ (infinite)
Focal length of a plane mirror = ∞ (infinite)

Sign conventions for reflection by spherical mirrors
(i) All distances are measured from the pole of a spherical mirror.
(ii) Distances measured in the direction of incident light are taken as positive. Distances measured in the direction opposite to that of the incident light are taken as negative.
(iii) The upward distances perpendicular to the principal axis are taken as positive, while the downward distances perpendicular to the principal axis are taken as negative.

Mirror formula- The relation between u, v and focal length (f) of a spherical mirror is known as mirror formula.
That is 1/u+ 1/v = 1/f.

Linear magnification- Linear magnification produced by a mirror is defined as the ratio of the size of the image to the size of the object. Denoted by m.
m = h’/ h = - v / u
Linear magnification produced by a plane mirror= + 1.

Refraction of light- The bending of light rays when they pass obliquely from one medium to another medium is called refraction of light.
A transparent medium through which light travels fast is known as optically rarer medium.
A transparent medium through which light travels slow is called as optically denser medium.

Laws of refraction-
(i) Incident ray, refracted ray and the normal to the surface separating two media all lie in the same plane.
(ii) The ratio of the sine of the incident angle ( <i) to the sine of the refracted angle ( < r) is constant.
Sin i/ sin r = constant.
This constant is known as the refractive index of second medium w.r.t. the first medium.
Absolute refractive index of a medium is defined as the ratio of the speed of light in vacuum ( c) to the speed of light in the medium ( v) . That is n = c/v
Relative refractive index of medium 2 w.r.t. the medium 1 is defined as the ratio of the speed of light in medium 1 to the speed of light in medium 2.
n 21 = n 2 / n 1
A medium whose refractive index is large is known as optically denser medium.
A medium whose refractive index is small is known as optically rarer medium.

Lenses are of two types: (i) Convex lens or converging lens- It is thick in the middle and thin at the edges.

(ii) Concave lens or diverging lens- It is thin in the middle and thick at the edges.
Convex lens converges the rays of light falling on it and acts as a magnifying glass.
Concave lens diverges the rays of light falling on it.
Principal axis of a lens is a line joining the centre of curvatures of two spherical surfaces forming a lens.
Optical centre of a lens is a point inside or outside a lens through which rays of light pass without deviation.
Principal focus of a lens is a point on the principal axis where all the rays of light parallel to the principal axis meet or appear to meet after refracting through the lens.
Focal length of a lens is the distance between the optical centre and the principal focus of the lens.
Focal length of a convex lens is positive.
Focal length of a concave lens is negative.

Lens formula = - 1/u + 1/v = 1/f.

Magnification produced by a lens : m = Size of image/ Size of object = h ‘ / h. Also m = v/u

Power of lens : P = 1/ f ( in m) = 100 / f (in cm )
Unit of power is dioptre (D).
Power of a lens is 1 dioptre if its focal length is 1 m or 100 cm.
Power of a convex lens is positive,Power of a concave lens is negative.

Image formation by a convex lens :

Human eye and colourful world
Human eye is like a camera.

Parts of human eye are- Cornea, Iris, Lens, Ciliary muscles, Retina, Optic nerve.
Cornea is the outermost part of eye. Iris controls the size of the pupil and pupil controls and regulates the entering the eye.
Lens of a human eye is a convex lens which focus the light on the eye to make the image of an object.
Ciliary muscles increases or decreases the focal length of the eye lens.
Retina acts as the screen or film to obtain the image of an object.
Optic nerve carries signal to the brain for interpretation.

Accomodation of an eye is the ability of eye lens to change its focal length to form sharp images of objects at different positions from the eye on the retina of the eye.

Near point of a normal human eye is 25 cm.

Far point is the farthest position of an object from a human eye so that the sharp image of the object is formed on the retina.
Range of vision is the distance between the near point and the far point of an eye.
For normal human eye , the range of vision is 25 cm to infinity

Power of accommodation of a normal human eyeis about 4 dioptre.

Defective eye – The eye which is unable to see the objects clearly is known as defective eye.
Common defects in a human eye are :
Hypermetropia or long sightedness or far -sightedness.
Myopia or short- sightedness or near- sightedness.
Presbyopia (iv) Astigmatism.

Hypermetropia- A human eye which can see distant object clearly but cannot see clearly the near objects is said to be suffered with a defect called hypermetropia.

Causes of hypermetropia- It is due to (i) increase in the focal length of eye lens (ii) the decrease in the size of eye ball.

Hypermetropia can be corrected by using a convex lens of suitable focal length.

Myopia- A human eye which can see near object clearly but cannot see clearly the distant objects is said to be suffered with a defect called Myopia.
Causes of Myopia- It is due to (i) elongation of the eye ball (ii) The excessive curvature of the cornea. Myopia can be corrected by using a concave lens of suitable focal lenght

Presbyopia- A human eye which cannot see clearly the near objects in old age is said to be suffered with a defect called Presbyopia.

Cause of Presbyopia- It arises because of the decrease in the flexibility of the human eye lens. Presbyopia can be corrected by using a bi-focal lens consisting of a concave and convex lens.

Astigmatism- A human eye which cannot focus on both horizontal and vertical lines simultaneously suffers from astigmatism.

Astigmatism can be corrected by using a cylindrical lens.

Cataract- A human eye in which a opaque membrane is formed over the eye lens suffers from cataract.

Cataract can be corrected by performing surgery.

Prism is a homogenous transparent refracting medium bounded by at least two non- parallel surfaces inclined at some angle.

Angle of prism- The angle between two non-parallel refracting surfaces is called angle of prism.

Angle of deviation- The angle between the directions of incident ray of light and the emergent ray of light is called angle of deviation of the ray of light passing through the prism.

Dispersion of white light- The phenomenon of splitting white light into seven colours when it passes through a glass prism is called dispersion of white light.

Spectrum- A band of seven colours of white light is called spectrum.
Red colour deviates the least while passing through a glass prism.
Violet colour deviates the most while passing through a glass prism.
Glass prism splits colours of white light passing through it and does not produce any colour by itself.

Rainbow is the example of dispersion of sunlight.
Rainbow is formed by tiny drops of water suspended in the atmosphere.

Atmospheric refraction- The refraction of light taking place in the atmosphere is known as atmospheric refraction.

Twinkling of stars takes place due to atmospheric refraction of light emitted by the stars.
When light falls on tiny particles , this light is absorbed by these particles. Then these particles re- emit light in all directions. This process is known as Scattering of light.
In a clear atmosphere of the earth, colours of small wavelengths like violet, and blue are scattered more than red colour. In a polluted atmosphere of the earth ( containing dust and smoke particles) , the scattering of colours of higher wavelengths is more than the scattering of smaller wavelengths.

 

♦ ANSWER THE FOLLOWING CONCEPTUAL QUESTIONS

1 What are the properties of the image formed by plane mirror?

2 One half of a convex lens is covered with a black paper.

(a) Show the formation of image of an object placed at 2F1, of such covered lens with the help of ray diagram. Mention the position and nature of image.

(b) Draw the ray diagram for same object at same position in front of the same lens, but now uncovered. Will there be any difference in the image obtained in the two cases? Give reason for your answer.

3 Draw a ray diagram to show refraction of light through a glass slab and label on it the following:

(i)Incident ray

(ii)Refracted ray

(iii)Emergent ray

(iv)Lateral shift (displacement)

4 If the image formed by mirror for all positions of the object placed in front of it is always virtual and diminished, state the type of the mirror. Draw a ray diagram in support of your answer. Where are such mirrors commonly used and why?

5 With the help of ray diagrams, show the formation of images of an object by a concave mirror. When it is placed
(i) beyond the center of curvature (ii) at the center of curvature.

6 Draw ray diagrams to show the formation of image by an object by a concave lens when the object is placed
(i) at infinity (ii) between infinity and optical Centre of the lens.

7 What is meant by the power of a lens? What is its SI unit? Name the type of lens whose power is positive. The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40cm from the lens, what is the nature and power of the lens? Draw ray diagram to justify your answer

8 A student has three concave mirrors A, B and C of focal lengths 20 cm, 15 cm and 10 cm respectively. For each concave mirror he performs the experiment of image formation for three values of object distance of 30 cm, 10 cm and 20 cm.

♦ Giving reason answer the following:

(a) For the three object distances, identify the mirror which will form an image equal in size to that of object. Find at least one value of object distance.

(b) Out of the three mirrors, identify the mirror which would be preferred to be used for shaving purpose.

(c) For the mirror B, draw ray diagram for image formation for any two given values of object distance.

9 Why does a light ray incident on a rectangular glass slab immersed in any medium emerge parallel to itself? Explain using a diagram.

10 Why do we prefer a convex mirror as a rear- view mirror in vehicles?

11 a) Which mirror is used by dentists?

b) What are the advantages and disadvantages of using convex mirror for seeing traffic at the rear?

12 Define the term absolute refractive index of a medium.

13 The refractive index of diamond is 2.42. What is the meaning of this statement?

14 Name the type of mirror used in following situations

a. Headlights of the car

b. Side/rear view mirror of a vehicle

c. Solar furnace

15 A spherical mirror produces an image of magnification -1 on a screen placed at a distance of 50 cm from the mirror.

(a) Write the type of mirror.

(b) Find the distance of the image from the object.

(c) What is the focal length of the mirror?

(d) Draw the ray diagram to show the image formation in this case.

♦ ANSWER THE FOLLOWING NUMERICALS

16 The refractive index of water is 1.33 and kerosene is 1.44. Calculate refractive index of the kerosene with respect to water.

17 State the laws of refraction of light. If the speed of light in vacuum is 3X108 m/s, find the speed of light in a medium of absolute refractive index 1.5.

18 If the speed of light in water is 2.25X108 m/s and the speed in vacuum is 3X108m/s. Calculate the refractive index of water.

19 A 5 cm tall object is placed perpendicular to the principal axis of a convex lens of focallength 20 cm. The distance of the object from the lens is 30 cm. Find the:

(i) position

(ii)nature

(iii)size of the image formed.

20 A 4.5 cm needle is placed 12 cm away from a convex mirror of focal length 15 cm. Give the location of image and magnification. Describe what happens to the image as the
needle is moved farther from the mirror.

21 A 5cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the nature, position and size of the image. Also find its magnification

22 (a) The refractive index of Ruby is 1.71. What is meant by this statement?

(b) The refractive index of some medium are given below:

Crown glass - 1.52
Water - 1.33
Sapphire - 1.77
In which of the medium is the speed of light

(i) maximum

(ii) minimum (iii) Calculate speed of light in sapphire.

23 It is desired to obtain an erect image of an object, using concave mirror of focal length of 12 cm.

(i) What should be the range of distance of an object placed in front of the mirror?

(ii) Will the image be smaller or larger than the object? Draw ray diagram to show the formation of image in this case,. Show the position of pole, principal focus and the Centre of curvature in the ray diagram.

24 List the new Cartesian sign convention for reflection of light by spherical mirrors. Draw a diagram and apply these conventions for calculating the focal length and nature of a spherical mirror which forms a 1/3 times magnified virtual image of an object placed 18 cm in front of it.

25 (a)An erect image 3 times the size of the object is obtained with a concave mirror of radius of curvature 36cm .What is the position of the object.

(b) When an object is placed at a distance 60 cm from convex mirror, the magnification produced is 1/2 .Where should the object be placed to get a magnification of 1/3?

26 List four characteristics of the image formed by a concave mirror of focal length 40cm when the object is placed in front of it at a distance of 20cm from its pole

 

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