(a) Between glass and water, glass is considered as a denser medium and water is rarer medium.

→ Because the velocity of light is less in glass as compared to air.

(b) When a coin is placed in a glass containing water it appears to rise a bit.

→ It is because of refraction of light. When light passes through rarer medium (air) to denser medium (water) it bends away from the normal and the coin rises a bit.

(c) When the fishes within the water are observed from the top of glass slab, the fishes appear to be slightly raised.

→ Due to the refraction of light. The rays bend while light passes through glass slab.

(d) Stars twinkle.

→ Stars twinkle due to refraction. Light rays coming from stars bends through atmosphere making stars seem to twinkle.

(e) The sun appears on the horizon about two minutes before the actual sunrise.

→ Because of the refraction of light.

(f) A diamond shines more at night.

→ Cut of the same shape diamond shines. Because diamond is cut in such a shape that total internal reflection occurs in it and it shines but glass does not because total internal reflection doesn’t occur in it.

(g) Sunlight is refracted when it is passed through a prism.

→ Because the sunlight goes from one medium to another medium through which it get refracted.

(h) A convex lens converges light rays.

→ A convex lens converges light rays because it is thick in the middle and thin at the edges which converge rays coming to it. It is also called convex lens.

(i) A concave lens diverges light rays.

→ Because it is thin at the middle and thick at the edges. So it is called diverging lens which diverges rays of light.

(j) Deficiency of vitamin A in the body is one of the main causes of night blindness.

→ Because vitamin A is essential for the proper functioning of retina and absence of it one have problem of night blindness.

(k) Color blindness occurs when the cone cells of the retina are defective.

→ Because the function of cone cells is to differentiate between colors.

1. What is the refraction of light?

→ The process of bending of light or changing the direction while passing through one optical medium to another medium is called the refraction of light.

2. Write the law of refraction of light.

→ The laws of refraction of light are:

• When light ray passes through one optical medium to another, the incident ray, normal and refracted ray lies in the same plane.
• When light ray passes from rarer to denser medium, it bends towards the normal and when light ray passes from denser to rarer it bends away from the normal.
• The ratio of sine angle of incidence to the sine angle of refraction remains constant for a pair of media. The constant value is refractive index of pair of medium which is denoted by μ.

3. What changes is noticed in the shape of a pencil that is half immersed in water as shown in the figure? Explain with ray diagram. Write the name of the process associated with the observations.

→ This process is called refraction of light. For example, refraction is seen while looking into water or fish tank.

4. When a light ray passes from water to air, the angle of incidence and angle of refraction formed at the water-air separating layer are 30° and 40° respectively. Draw a ray diagram showing the water-air interface and explain how a pencil out of water appears farther away from its actual position when an observer is inside the water.

5. What is critical angle?

→ The angle of incidence in the denser medium under the condition which the value of its corresponding angle of refraction in the rarer medium becomes 90° is known as the critical angle.

6. What is the total internal reflection of light?

→ The phenomena reflection of light to the same medium when it is passed through a denser medium to a rarer medium is called total internal reflection of light.

7. Write two conditions necessary for the total internal reflection of light.

8. At present, data can be transmitted at a very high rate through fibre optic cable. Mention the role of total internal reflection of light in fibre optic.

→ The role of total internal reflection of light in fibre internet is that by the use of total internal reflection, light and transmitted through optic fibre which allows for the high speed propagation.

9. In endoscope, colonoscopy and key hole surgery how is total internal reflection of light applicable to the doctors and is used in the human body without incisions?

10. What is dispersion of light?

→ The process in which light is falling on the prism and splits it into seven colours while passing through the object is known as the dispersion of light.

11. Mention the reason for the dispersion of light.

→ The reason is that when a light ray enters a prism and exit from it into the surroundings, since as a result the light rays of different velocities that have separated inside the prism, exit by bending towards the base and falling on the screen at different places with a range of colours from red to [Unreadable in source].

(i) Centre of curvature

→ The centre of curvature is the centre of the sphere where forms the curvature point of the lens.

(ii) Optical centre

→ The optical centre is the geometrical centre of the lens where the principal axis intersects the lens surface.

(iii) Principal axis

→ Principal axis is the line passing through the optical centre and two centres of curvature of the lens.

(iv) Focus

→ Focus of the lens is the point where light rays converge after passing through a lens or reflecting off a mirror.

15. What is meant by the power of a lens?

→ Power of a lens is its ability to converge or diverge the rays of light falling on it.

16. The power of two convex lenses are +2D and +4D respectively.

(i) Which of them is thicker? Give reason.

→ The 4D lens is thicker because it has a shorter focal length.

(ii) Calculate the focal length of both lenses.

17. In which case is the image formed by a convex lens one real and of the same size as the object? Show the ray diagram.

→ An image formed by a convex lens will be real and same size of the object if the object is placed at twice the focal length (2F).

18. Why the spear thrown by the man shown in the figure does not hit the target? How can he hit the target? Explain it on the basis of internal reflection.

→ The spear thrown by the man shown in figure does not hit the fish because the water will not hit the fish. In water because, due to water and caused reflection of light while passing from water to denser medium, the fish is not at its actual position as rays refracted from actual side.

19. What process is shown in the ray diagram? Name two devices that operate on this process.

→ Total internal reflection is shown in this diagram.

The two devices that operate this process are:

• Router connector.
• Endoscope optical fibre.

20. Complete the ray diagram by copying the given diagram to answer sheet.

22. The given figure shows the dispersion of a light ray through a triangular prism. Answer the following questions based on the given ray diagram:

(i) Which colours of lights were indicated by X and Y?

→ X indicates colour violet whereas Y indicates colour red.

(ii) Write down the reason why it is bent less than X when the light rays comes out of the prism.

→ Because the wave length of red is greater than violet. So as the wavelength is lower, the refractive index increases and as the refractive index increases the angle of deviation also decreases. This is why red colour is less bent than violet.

23. How can the light dispersed through prism be converted back into white light?

→ By making the prism upside down, the light dispersed by a prism can be converted back into white light.

24. The focal lengths of the two lenses are 20cm and −20cm respectively. Mention the type of lenses too and draw ray diagram.

→ The lens with focal length +20cm is a convex lens and −20cm is a concave lens. The convex lens is thick in middle and convex lens form upright and magnified image.

(i) Ciliary muscle

→ It controls the shape of the lens, allowing it to adjust the focus for near and far vision.

(ii) Cornea

→ It protects the sensitive inside and helps to focus incoming light onto the retina.

(iii) Lens

→ It helps to focus light onto the retina by changing its shape through the action of ciliary muscle.

(iv) Pupil

→ It regulates the amount of light entering the eye.

(v) Retina

→ The function of retina is to detect light and convert it into electrical signals that are sent to the brain where they are processed to form visual images.

(vi) Iris

→ Its function is to regulate the size of the pupil.

26. Write any two problems that may be seen in cornea injury.

→

• Blurred or distorted vision.
• Pain and sensitivity to light.
• Shortsightedness.
• Corneal ulcer.

27. Explain the role of the ciliary muscle in the change in the thickness of the eye lens when a student studies in a classroom within his eye from the distance nearby compared to a distant object seen out the window.

→ The ciliary muscle plays a crucial role in the process of accommodation which allows the eye to focus on objects at different distances. When looking at the whiteboard which is farther the ciliary muscle contract which enables better focus on nearby objects and when student focus to outside distant object the ciliary muscle relaxes which is placed for focusing on nearby object.

28. Identify the type of defect of vision shown in the eye in the diagram. Write two causes of the defect along with its correction.

→ This type of defect of vision is longsightedness. The two cause of this are:

• shortened eyeball.
• flattened cornea.

29. An image in the class has difficulty in seeing the letters copied on the white board when he sits on last bench. Based on this answer the following questions:

(i) What type of defect of vision does the student have?

→ The student have short-sightedness.

(ii) Draw ray diagram showing his type of defect of vision of the student.

(iii) Write any two reasons that this defect may be caused by.

• elongation of eyeball
• curvature of the cornea.

(iv) Explain with a ray diagram the role of the lens used to correct this defect.

30. Explain with a ray diagram the role of the lens used to correct long-sightedness.

→ To correct long-sightedness, a converging lens also known as a convex lens is used. In a ray diagram, some of rays of distant object are refracted by the convex lens. The lens brings the rays closer together causing them to converge. The converging effect helps to compensate for the impaired inward light caused by longsightedness allowing the image to form correctly on the retina.

31. A student concludes that, the effect of defect of vision is more on a person wearing spectacles with thick lenses than one wearing thick lenses. Are the above conclusion correct? Justify with appropriate reasons.

→ The statement is correct because thicker lenses are usually prescribed for more severe vision defects. This means lens typically indicates a greater defect than thin lenses.

32. Compare the use of spectacles and use of contact lenses to correct vision defects.

33. Explain the laser surgery method used to restore eyesight by cataract.

34. What is a cataract? Write the role of the intra ocular lens developed by Dr. Sanduk Ruit.

→ A cataract is a clouding of the natural lens in the eye that causes blurry or impaired vision. It’s a common age-related condition but can also be caused by factors like genetics, injury or medical conditions.