Short Notes: Refraction

                                 REFRACTION

Bending of light when it enters obliquely from one transparent medium to
another.

• Speed of light is maximum in vaccum. It is 3 × 108 m/s.
• Cause of refraction : Change in speed of light.
• Some examples of refraction :
(i) The bottom of swimming pool appears higher.

(ii) A pencil partially immersed in water appears to be bent at the interface of water and air.

(iii) Lemons placed in a glass tumbler appear bigger.
(iv) Letters of a book appear to be raised when seen through a glass slab.

Refraction through glass slab

• The extent of bending of ray of light at the opposite parallel faces of rectangular glass slab is equal and opposite, so the ray emerges parallel to incident ray.

• Lateral displacement depends on :

(a) Refractive index of glass slab
(b) Thickness of the glass slab

 

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Laws of Refraction:-

(i) The incident ray, the refracted ray and the normal to the interface of two
transparent media at the point of incidence, all lie in the same plane.

Rules for image formation by convex lens:-

(i) A ray of light parallel to principal axis of a convex lens always pass through the focus on the other side of the lens.

(ii) A ray of light passing through the principal focus will emerge parallel to principal axis after refraction.

(iii) A ray of light passing through the optical center will emerge without any deviation.

Ray Diagrams of Imaged formed by Convex Lens

(i) When object is at infinity :

Image
Position − At ‘F2’
Nature – Real, inverted
Size – Point sized or highly diminished
(ii) When object is beyond ‘2F1’

Image
Position – Between ‘F2’ and ‘2F2’
Nature – Real, inverted
Size – Diminished
(iii) When object is at ‘2F1’

Image
Position – At ‘2F2’
Nature – Real, inverted
Size – Same size
(iv) When object is between ‘F1’ and ‘2F1’

Image
Position – Beyond ‘2F2’
Nature – Real, inverted
Size – Enlarged

(v) When object is at ‘F1’

Image
Position – At Infinity
Nature – Real, inverted
Size – Highly enlarged

(vi) When object is between ‘F1’ and optical centre

Image
Position – On the same side of the lens as object
Nature – Virtual and erect
Size – Enlarged

 

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Rules for Image Formation by Concave Lens

(i) A ray of light parallel to the principal axis appear to diverge from the principal focus located on the same side of the lens.

(ii) A ray of light appearing to meet at the principal focus of a concave lens will emerge parallel to principal axis.

(iii) A ray of light passing through the optical centre of a lens will emerge without any deviation.

Ray Diagrams of Images Formed by a Concave Lens

(i) When object is placed at infinity :

Image
Position − At ‘F1’
Nature – Virtual, erect
Size – Point sized or highly diminished

(ii) When object is placed between infinity and optical centre


Position – Between ‘F’ and ‘O’
Nature – Virtual, erect
Size – Diminished

Sign convention for spherical lenses

• Sign conventions are similar to the one used for spherical mirrors, except
that measurements are taken from optical center of the lens.
• Focal length of convex lens = Positive
Focal length of concave lens = Negative


CHECK YOUR KNOWLEDGE: 

 

1. Refractive indices of medium A, B and C are 1.3, 1.5 and 1.4 respectively.

In which of the following the speed of light will be the :
(a) fastest ?
(b) slowest ?
2. A compound lens is made up of two thin lenses having power + 12.5 D and – 2.5 D. Find the focal length and power of the combination.
3. Light enters from air to kerosene having a refractive index of 1.47. What is the speed of light in kerosene ?
4. A 5 cm tall object is placed perpendicular to principal axis of a convex lens of focal length 10 cm. If the object is placed 30 cm away from the lens, find the position, size and nature of image.
5. One half of a convex lens is covered with black paper.
(a) Show the formation of image of a object placed at 2F, of such covered lens with the help of ray diagram. Mention the position and nature of the 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 image obtained in the two cases ? Give reasons for your answers.
6. A thin converging lens forms a (i) real magnified image, (ii) virtual magnified image.
(a) Write the position of object in each case.
(b) Draw labelled diagram for each case.
7. (a) What happens to a ray of light when it travels from one medium to another having equal refractive indices ?
     (b) State the cause of refraction of light.

8. (a) Define 1 dioptre of power. Find the focal length of a lens of power – 2.0 D.
(b) Why does a lemon kept in water in a glass tumbler appear to be bigger than actual size ?
9. A ray travelling in water enters obliquely into glass. Does the light bend towards or away from the normal and why ?
10. An object is placed at the focus of a convex lens. Draw ray diagram to locate the position of image formed.

Hints to Long Answer Type Questions

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