Practice Questions: Ray Optics - Light : Reflection and Refraction

Fill in the Blanks

Q1: When light passes from a rarer to a denser medium, it bends _______ the normal.

Ans: towards

Explanation: When light travels from a rarer medium (for example, air) into a denser medium (for example, glass), its speed decreases. Because of this change of speed the direction of the light ray changes and the ray bends towards the normal at the point of incidence.

Q2: The point at which the incident ray meets the mirror is called the _______ point.

Ans: incident 

Explanation: The point of incidence is the exact point on the reflecting surface where the incoming (incident) ray strikes. The normal is drawn at this point to measure the angles of incidence and reflection.

Q3: The angle between the incident ray and the normal is known as the angle of _______.

Ans: incidence

Explanation: The angle of incidence is the angle formed between the incident ray and the normal (a line perpendicular to the surface) at the point of incidence.

Q4: The type of mirror used in a magnifying glass is a _______ mirror.

Ans: convex 

Explanation: The blank here suggests a mirror. The term convex refers to a surface curved outwards. However, in correct optical practice a magnifying glass is not a mirror but a convex lens. Convex mirrors are used where a wide field of view is needed (for example, vehicle side mirrors), while a magnifying glass uses a convex lens to produce an enlarged virtual image of a nearby object.

Q5: The phenomenon responsible for the dispersion of white light into its component colours is called _______.

Ans: dispersion 

Explanation: Dispersion is the splitting of white light into its constituent colours (spectrum) because different wavelengths refract by different amounts when passing through a medium (for example, through a prism). Each wavelength has a different refractive index and so they emerge separated in direction.

Short Answer Questions

Q6: Define the term 'refraction' of light.

Ans: Refraction is the bending of light when it passes from one transparent medium to another, due to a change in its speed.

Explanation: When a light ray crosses the boundary between two media with different optical densities (for example, air to water), its speed changes and the ray changes direction. The amount of bending depends on the refractive indices of the two media and the angle of incidence.

Q7: State Snell's Law.

Ans:  

Snell's law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for light passing between two given media.

Mathematical form: n₁ sin α₁ = n₂ sin α₂, where n₁ and n₂ are the refractive indices of the first and second media respectively, and α₁, α₂ are the angles of incidence and refraction measured from the normal.

Q8: Explain the term 'total internal reflection' of light.

Ans: 

• Total internal reflection occurs when a light ray travels from a denser medium to a rarer medium and strikes the boundary at an angle greater than a certain limiting angle called the critical angle.
• For angles greater than the critical angle, the ray is completely reflected back into the denser medium and none of the light is refracted into the rarer medium.
• The critical angle θ_c satisfies sin θ_c = n₂/n₁, where n₁ is the refractive index of the denser medium and n₂ that of the rarer medium (with n₁ > n₂).
• Applications include optical fibres, prisms used in binoculars, and some types of optical sensors.

Q9: Differentiate between a real image and a virtual image.

Ans: 

• Real image: Formed when actual rays of light converge to meet at a point after reflection or refraction. A real image can be projected on a screen. It is usually inverted with respect to the object.
• Virtual image: Formed when the outgoing rays appear to diverge from a point behind the mirror or lens; the rays do not actually meet there. A virtual image cannot be captured on a screen and is usually erect. Examples include the image seen in a plane mirror and the enlarged image seen through a magnifying glass (when the object is within the focal length).

Q10: Why does a pencil appear to be bent when placed in a glass of water?

Ans: The pencil appears bent due to the phenomenon of refraction. When light from the part of the pencil immersed in water passes from water (denser medium) into air (rarer medium), it bends away from the normal. Because our eyes trace light rays in straight lines, the apparent position of the submerged part shifts; this makes the pencil look bent at the water surface.

Long Answer Questions

Q11: Explain how a convex lens forms an image of an object.

Ans: 

Working principle: A convex lens (converging lens) refracts parallel rays of light so that they meet (converge) at a point called the principal focus on the other side of the lens. The distance from the optical centre of the lens to the principal focus is the focal length.

Ray behaviour and principal rays:

• A ray parallel to the principal axis refracts through the lens and passes through the principal focus on the opposite side.
• A ray passing through the optical centre of the lens continues in a straight line without deviation (to a first approximation).
• A ray passing through the principal focus on the object side emerges parallel to the principal axis after refraction by the lens.

Image formation for different object positions:

• If the object is placed beyond twice the focal length (beyond 2F): the image is real, inverted, smaller than the object, and formed between F and 2F on the other side.
• If the object is placed at twice the focal length (at 2F): the image is real, inverted, same size as the object, and formed at 2F on the other side.
• If the object is placed between F and 2F: the image is real, inverted, and larger than the object; it is formed beyond 2F.
• If the object is at the principal focus F: refracted rays emerge parallel and the image is formed at infinity (no finite image).
• If the object is placed within the focal length (between the lens and F): the image is virtual, erect, and magnified; it appears on the same side of the lens as the object. This is the principle used in magnifying glasses.

Q12: Describe the difference between regular and diffuse reflection.

Ans: 

• Regular reflection: Occurs from smooth, polished surfaces (for example, plane mirrors). Parallel incident rays remain parallel after reflection and produce clear, well-defined images.
• Diffuse reflection: Occurs from rough or irregular surfaces (for example, paper, unpolished wood). Parallel incident rays are scattered in many directions, so no clear image is formed; instead we see the surface uniformly illuminated.

Q13: Why does the sky appear blue to us?

Ans: 

• The sky appears blue mainly due to Rayleigh scattering, which is the scattering of sunlight by molecules and very small particles in the Earth's atmosphere.
• Light of shorter wavelengths (blue and violet) is scattered much more strongly than light of longer wavelengths (red). The amount of scattering varies approximately as 1/λ⁴, where λ is the wavelength.
• Although violet light is scattered even more than blue, our eyes are more sensitive to blue and some violet light is absorbed by the upper atmosphere, so the sky appears predominantly blue.

Q14: Explain the formation of a rainbow.

Ans:  

A rainbow forms when sunlight interacts with spherical raindrops in the atmosphere. The process in each drop involves three main steps:

• Refraction on entry: sunlight entering a raindrop slows down and bends; different colours refract by different amounts (dispersion).
• Internal reflection: the refracted light reflects from the inner surface of the raindrop.
• Refraction on exit: the reflected light refracts again as it leaves the drop, further separating the colours.

The separation of colours (dispersion) causes red light to emerge at a larger angle relative to the incoming sunlight (about 42°) and violet at a slightly smaller angle (about 40°) for the primary rainbow, producing the red-on-top, violet-on-bottom primary arc. A secondary rainbow (if present) is formed by two internal reflections and has reversed colours and a larger radius.

Q15: How does the human eye focus on near and distant objects?

Ans: 

The human eye focuses using the process called accommodation. Accommodation changes the shape and optical power of the eye lens so images of objects at different distances are formed clearly on the retina.

• To focus on a near object: the ciliary muscles contract, causing the suspensory ligaments to relax; the lens becomes thicker and more convex, increasing its refractive power so that rays from a near object converge on the retina.
• To focus on a distant object: the ciliary muscles relax, the suspensory ligaments pull the lens flatter, reducing its curvature and refractive power so that rays from distant objects focus on the retina.
• Normal range: the far point of a normal eye is at infinity and the near point is typically about 25 cm for a young adult. Defects such as myopia and hypermetropia occur when the eye cannot properly form images on the retina; corrective lenses restore the required focus.

Summary

This set of practice questions covers fundamental ideas in ray optics including reflection, refraction, Snell's law, dispersion, image formation by lenses and mirrors, total internal reflection, and common atmospheric optics phenomena such as the blue sky and rainbows. Understanding the laws of reflection and refraction, the behaviour of rays through lenses, and the dependence of optical effects on wavelength will help in solving related numerical and conceptual problems.