Optics MCQ Quiz - Objective Question with Answer for Optics - Download Free PDF

The correct answer is refraction of light.

Key Points

• The twinkling of stars is primarily caused by refraction of light as it passes through different layers of the Earth's atmosphere.
• As the light from a star enters the Earth's atmosphere, it gets bent multiple times due to varying air densities, resulting in a change of direction.
• This constant bending of light rays makes the stars appear to twinkle or flicker from our point of view on Earth.
• Refraction is the bending of light as it passes from one medium to another with different densities, such as from space (vacuum) into the Earth's atmosphere (air).

Additional Information

• Interference of Light
  • Interference is a phenomenon where two or more light waves superimpose to form a resultant wave of greater, lower, or the same amplitude.
  • It is responsible for patterns such as fringes seen in experiments like the double-slit experiment.
• Reflection of Light
  • Reflection occurs when light bounces off a surface back into the original medium.
  • It is described by the law of reflection, which states that the angle of incidence is equal to the angle of reflection.
• Dispersion of Light
  • Dispersion is the separation of light into its constituent colors due to different degrees of refraction for different wavelengths.
  • This phenomenon is observed when light passes through a prism, creating a spectrum of colors.

The correct answer is virtual image.

Key Points 

• An image that cannot be obtained on a screen is called a virtual image.
• Images are of two types: real images and virtual images.
• Real images are formed on a screen, whereas virtual images cannot be obtained on the screen. 

Difference between real image and virtual image:

The correct answer is Reflection.

• The property of reflection is used in making a periscope.

Key Points

• Reflection of Light:
  • The return of light into the same medium after striking a surface is called reflection.
  • There are two laws of reflection:
    • The angle of incidence is always equal to the angle of reflection.
    • The incident ray, normal at the point of incidence and reflected ray, all lie in the same plane.
• Reflection From Plane Mirror:
  • The image is virtual and laterally inverted.
  • The size of the image is equal to that of the object.
  • The distance of the image from the mirror is equal to the distance of the object from the mirror.
  • If an object moves towards (or away from) a plane mirror with speed u, relative to the object, the image moves towards (or away) with a speed of 2u.
• Periscope is used in submarines for navigation and to enable an observer to see his surroundings while remaining undercover, behind armour, or submerged.
• It uses two plane mirrors at 45⁰ angles facing each other and works on the principle of reflection.

Additional Information

• Refraction of light:
  • When a ray of light propagating in a medium enters the other medium, it deviates from its path. This phenomenon of change in the direction of propagation of light at the boundary, when it passes from one medium to another medium, is called the refraction of light. 
• Dispersion of Light:
  • When a ray of white light (or a composite light) is passed through a prism, it gets split into its constituent colours. This phenomenon is called the dispersion of light.
  • The coloured pattern obtained on a screen after the dispersion of light is called a spectrum.

The correct answer is Concave.

• Concave mirrors have reflecting surface curved inwards.
  • ​Concave mirrors reflect light inward to one focal point.
  • They are used to converge light.
  • Unlike convex mirrors, concave mirrors show different image types depending on the distance between the object and the mirror.

Additional Information

• A convex mirror is a curved mirror where the reflecting surface is oriented towards the light source.
• A concave mirror is a curved mirror where the reflecting surface is curved inward.
• A curved mirror is a mirror with a curved reflective surface.
  • The surface may be bent inward (concave mirror) or maybe raised outward (convex mirror).

Calculations:

Given,

Distance of the object from the lens = u = -10 cm

Refractive index of the lens = µ = 1.5

The Radii of curvature of the lens are 20 cm in magnitude

R₁ = 20 cm and R₂ = -20 cm          (As per sign convention)

According to Len's Maker's formula

\(\frac{1}{f}=(\mu - 1)(\frac{1}{R_1}-\frac{1}{R_2})=(1.5-1)(\frac{1}{20}-\frac{1}{-20})\\ =0.5 \times \frac{2}{20}=\frac{1}{20}\\ or, \; f=20 \; cm \)

From the Lens equation,

\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{v}=\frac{1}{f}+\frac{1}{u}\\ or, \; v=\frac{fu}{u+f}=\frac{20 \times (-10)}{-10+20}=\frac{-200}{10}=-20 \; cm\)

The image is formed 20 cm on the same side as the object.

• The correct answer is option 3 i.e. Total internal reflection of light.

Explanation:

• On hot summer days, the air near the ground becomes hotter than the air at higher levels.
• The refractive index of air increases with its density. Hotter air is less dense and has a smaller refractive index than cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height.
• As a result, light from a tall object such as a tree passes through a medium whose refractive index decreases towards the ground.
• Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle. 

• To a distant observer, the light appears to be coming from somewhere below the ground and naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called a mirage.
• This type of mirage is especially common in hot deserts.
• Some of you might have noticed that while moving in a bus or a car during a hot summer day, a distant patch of road, especially on a highway, appears to be wet. But, you do not find any evidence of wetness when you reach that spot. This is also due to mirage.

Confusion Points

• Students are usually confused about whether mirage is the phenomenon of refraction or total internal reflection.
• It is a case of total internal reflection.

 CONCEPT:

Explanation:

• When the eye lens becomes less elastic then it happens.
• When the Objects nearby appear to be blurred to a person then it is called Presbyopia.
• Due to presbyopia, the ciliary muscles of the eye lens becomes weak and the visual defect started. 
• So option 2 is correct.

Explanation:

• Dispersion is the phenomenon by which white light splits into its 7 constituent colours while moving from one transparent medium to another.
• The 7 colours are red, orange, yellow, green, blue, indigo and violet.
• Rainbow is formed due to the dispersion of sunlight as it passes through droplets of water in the air.
•  A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms.
• They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.
• Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.

Additional Information
Reflection 

• The phenomena in which light ray is sent back into the same medium from which it is coming, on interaction with boundary, is called reflection.
• Laws of reflection: it states that, if a light ray is reflected from a plane flat polished surface its angle of the incident will always be equal the angle of reflection

 i.e., The angle of incidence (θ i ) = Angle of reflection (θ r )

Refraction

• The bending of a ray of light when it goes from one medium to another is called refraction of light.
  • When light enters from a rarer to a denser medium it bends towards the normal to the interface of the media.
  • Similarly, when light travels from a denser to the rarer medium it bends away from the normal.
• In refraction through a glass slab, the ray that first falls on the slab is incident ray, when it enters the slab it is called refracted ray and the outcoming ray is called an emergent ray.

The following diagrams shows the refraction of light through a glass slab:

• Applying the above-given principles it can be observed that emergent ray is parallel to the incident ray (here, AE is parallel to CD). 

Diffusion:

• When the light rays are reflected from a piece of paper and the reflected beam is scattered. Then this phenomenon is called diffusion of light.

• The focal length of the mirrors does not depend on the refractive index of the medium.
• The incident ray angle and angle of reflection will remain the same as long as the immersed object is not lens. Therefore, the focal length will remain the same as F.

The correct answer is two plane mirrors.

Key Points

• A periscope makes use of two plane mirrors.
• Periscopes are used in order to see the objects that are not in direct line of sight.
• Its work is based on the laws of reflection.
• Uses:
  • It is used by the submariners to view the objects at the surface of the water.
  • It is used in a nuclear reactor to observe the chemical reactions that are taking place.
  • In the military periscopes are used to observe from their hiding position​.

The correct option is 4 i.e. Diminished, virtual, and erect image.

Key Points

The rearview mirrors of the vehicle should produce diminished, virtual and erect image.

• The mirror in the rearview is a convex mirror which provides.
•  Convex mirrors bulge outwards and provide a wide field of view.

• Uses of Convex Mirror:

  • hallways of buildings stores, schools, hospitals.

  • used automated teller machines as a handy security feature that allows users to see what all is happening behind them.

• A concave mirror reflects the light falling on it to one focal point thus creating a reflective image of the object.

• It is used in the headlights of vehicles.
• Some uses Of Concave Mirror:
  • Shaving mirrors.
  • Head mirrors.
  • Astronomical telescopes.
  • Headlights.
  • Solar furnaces.

• The power of a lens is given by \(P = \frac{1}{f}\) 
• Where f is the focal length of the lens.
• In the case of a concave lens, the power will always be negative as the focal length is negative.
• So, \(P = \frac{1}{-5} = -0.2D\)

The correct answer is 1.2 × 108 m/s.

Key Points

CONCEPT:

• Refractive index (μ): The ratio of the velocity of light in vacuum to the velocity of light in the medium is called refractive index of that medium.

\(\text{The refractive index of a substance/medium}=\frac{\text{Velocity of light in vacuum}}{\text{Velocity of light in the medium}}\)

So μ = c/v

Where c is the speed of light in vacuum and v is the speed of light in the medium.

CALCULATION:

Given that:

Refractive index of the diamond (µ_d)= 2.5

We know

The velocity of light in vacuum (c) = 3 × 10⁸ m/s

To find the velocity of light in diamond (v)

Now,

\(μ _d=\frac{c}{v}\\ or, \; 2.5= \frac{3 \times 10^8}{v}\\ or, \; v=\frac{3 \times 10^8}{2.5}=1.2\times 10^8 \; m/s\)     

Hence option 1 is correct.   

Convex mirror or diverging mirror is a type of curved mirror that reflects the light outwards and hence form virtual images.

The convex mirror is generally used as a rearview mirror because it provides the maximum rear field of view and image formed is always erect.

Additional information:

1. A plane mirror always forms a virtual image (behind the mirror). The image and object are the same distance from a flat mirror, the image size is the same as the object size, and the image is upright.
2. Concave mirror enlarged and erect image that is why it is used as a shaving mirror and by doctors to examine eye, nose or ear.