Geometric Optics Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. Define optics. What are the types of optics?

Ans. Optics is the branch of physics that deals with the nature, production, and propagation of light.

There are major two types of optics

• Geometrical optics or Ray optics
• Physical optics or Wave optics

Ques. What is Geometrical optics?

Ans. Geometrical optics is based on the rectilinear propagation of light and deals with reflection and refraction of light, and the formation of images by mirrors, prisms, and lenses.

Ques. The range of wavelength of visible light is

1. 100 nm to 200 nm
2. 250 nm to 300 nm
3. 300 nm to 350 nm
4. 400 nm to 700 nm

Ans. The correct option is d. 400 nm to 700 nm

Explanation: Light is a form of energy, defined as the visible part of the electromagnetic spectrum of wavelength ranging from 400 nanometers to 700 nanometers.

Ques. The spectrum of visible light is emitted and absorbed in the form of

1. Neutrons
2. Protons
3. Photons
4. Electrons

Ans. The correct option is c. Photons

Explanation: Photon is a small packet of electromagnetic radiation. The spectrum of visible light is emitted and absorbed in the form of photons.

Ques. The separation of white light into different colors is known as.

1. Refraction
2. Reflection
3. Total internal reflection
4. Dispersion

Ans. The correct option is d. Dispersion.

Explanation: Dispersion is the process of splitting white light into its constituent colors while passing through a prism.

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Short Answers Questions [2 Marks Questions]

Ques. What is the limitation of refracting-type telescopes?

Ans. Following are the limitations of refracting-type telescopes

1. The refracting-type telescopes suffer from spherical and chromatic aberrations.
2. The objective lens of a very large aperture is very difficult to manufacture.

Ques. What does VIBGYOR mean?

Ans. VIBGYOR represents the colors of the visible spectrum of light. Each letter of VIBGYOR represents a separate color of visible light.

• V stands for Violet
• I stand for Indigo
• B stands for Blue
• G stands for Green
• Y stands for Yellow
• O stands for Orange
• R stands for Red

Ques. What are the various optical properties of light?

Ans. The various optical properties of light are

• Reflection
• Refraction
• Polarization
• Absorption
• Transmission
• Dispersion

Ques. Define total internal reflection of light.

Ans. The phenomenon by virtue of which a ray of light traveling from a denser medium to a rarer medium is sent back to the same denser medium provided it strikes the interface of denser and rarer media at an angle greater than the critical angle is called total internal reflection of light.

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Long Answers Questions [3 Marks Questions]

Ques. Describe the phenomenon behind the twinkling of stars.

Ans. The light emitted by a star is reflected continuously by the different layers of the atmosphere before it reaches the Earth. Due to the repeated refraction of light through the atmosphere, the apparent position of the star is different from its actual position. Since the temperature and density of the atmosphere are continuously changing and the atmosphere is mobile, so the apparent position of the stars also changes continuously. This continuous change in the apparent position of a star leads to the twinkling of the star.

Ques. Why is the Sun seen a few minutes before it actually rises?

Ans. The sun appears to rise about 2 minutes before the actual sunrise because of the refraction of light due to the denser atmosphere near the earth.

Due to the refraction of light, the apparent shift in the direction of the sun is about 1/2 degree leading to a delay of about 2 minutes in actual and apparent sunset.

Earth completes one rotation i.e. 360 degrees about its axis in 24 hours. Therefore, 

Total time for 360⁰ shift = 24 hours

Time taken for 1/2⁰ shift = (24/360) x (1/2) = 1/30 hour = 2 minutes.

Ques. Define the phenomenon of Scattering of light and state Rayleigh’s law of scattering.

Ans. The process of re-radiating the light by atoms or molecules of gases in all directions is known as the scattering of light.

• When sunlight enters the atmosphere of the earth, the light is absorbed by the atoms and molecules of the gases present in the earth’s atmosphere.
• These atoms and molecules re-emit light in all directions.
• The atom or molecular or any other particle which scatters the light is known as a “Scatter”.
• Scattering light depends upon the size of the scatters in the atmosphere.

Rayleigh’s law of scattering: According to Rayleigh, the intensity of the scattered light intensity of scatted light is inversely proportional to the fourth power of the wavelength of the incident light, provided the size of the particles scattering light is very-very small as compared to the wavelength of the incident light. I.e.

The intensity of scattered light ∝ 1/λ⁴

Where λ is the wavelength of light.

The larger the wavelength, the lesser is scattering of light.

Ques. Write applications of concave and convex mirrors.

Ans. The application of concave mirrors are

• Concave mirrors are preferred over plane mirrors for shaving and makeup.
• Concave mirrors are used by dentists, eye specialists, and ear, nose, and throat (ENT) specialists, to focus light on teeth, eyes, etc. to examine these organs.
• Concave mirrors are used in reflecting-type telescopes.

The applications of convex mirrors are

• A convex mirror is used as a rear view mirror in the vehicles.
• It is used as a reflector for street lightening purposes.
• It is used in big shops to minitor the activities of customers.

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Very Long Answers Questions [5 Mark Questions]

Ques. Write the characteristics of the image formed by a plane mirror.

Ans. The characteristics of the image formed by a plane mirror are

• The size of the image formed by a plane mirror is the same as that of the object.
• The image formed by a plane mirror is at the same distance behind the mirror as the object in front of it.
• The image formed by a plane mirror is erect, virtual, and laterally inverted.
• If an object moves towards or away from the plane mirror with a speed v, then the image of the object will move towards or away from the mirror with a speed of 2v.
• The minimum length of the plane mirror required to have the full length of a person standing in front of it is equal to half the height h of the person.
• For a given incident ray, if the mirror is rotated through an angle θ, then the reflected ray turns through an angle of 2θ.

Ques. A glass is filled with water up to 12.5 cm. The apparent depth of an object lying at the bottom of a glass is measured by a microscope to be 9.4 cm. Calculate the refractive index of water. If water is replaced by a liquid of refractive index 1.63 up to the same height, by what distance would the microscope have to move to focus the object again?

Ans. Given

• Real depth = 12.5 cm
• Apparent depth = 9.4 cm

The refractive index of water is given by

ƞ_water = real depth / apparent depth = 12.5 / 9.4 = 1.33

Now, When water is replaced by a liquid

• Refractive index of liquid, ƞ_liquid = 1.63 (Given)
• The height of the liquid will be the same as the previous i.e. real depth = 12.5 cm

Therefore, the apparent depth is given by

Apparent depth = real depth / refractive index (ƞ_liquid)

⇒ Apparent depth = 12.5 / 1.63 = 7.66 cm

The distance through which the microscope is to be moved = 9.4 - 7.66 = 1.74 cm.

Ques. A small object is at a distance of 5 cm from a convex lens with a focal length of 6 cm. Calculate

1. The position of the image.
2. Linear magnification produced by the lens
3. Magnifying power of the lens

Ans. Using the sign convention for the given values, we have

• Object distance from the lens, u = - 5 cm
• The focal length of the convex lens, f = +6 cm

1. Using lens formula, 1/f = - 1/u + 1/v, we get

1/v = 1/f + 1//u

Where

• v = distance of the image from the lens
• u = distance of the object from the lens
• f = focal length of the lens

On substituting the values, we get

1/v = 1/6 - 1/5 = - 1/30

⇒ v = – 30 cm

Therefore, the position of the image is 30 cm in front of the lens.

2. Linear magnification is given by

m = distance of the image / distance of the object = -30 / -5 = 6 dioptre.

100. Magnifying power of the lens is given by

MP = (Least distance of distinct vision) / (distance of the object) = -25 / -5 = 5

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