Chapter 9: Ray Optics and Optical Instruments
Topics Covered
- Refraction of light, total internal reflection and its applications, optical fibers
- Refraction at spherical surfaces, lenses, thin lens formula, lens maker’s formula
- Magnification, power of a lens, combination of thin lenses in contact
- Refraction of light through a prism
- Optical instruments: Microscopes and astronomical telescopes (reflecting and refracting) and their magnifying powers
MCQs
- Click here to Calculate you answer : Mirror Formula Calculator : Find Image Distance, Object Distance or Focal Length
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1. A convex mirror of focal length 10 cm forms an image five times smaller than the object. The distance of the object is:
- (a) 50 cm
- (b) 40 cm
- (c) 30 cm
- (d) 60 cm
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2. A pencil of length 10 cm is held parallel to the principal axis of a concave mirror of radius of curvature 20 cm. The distance of one of the ends of the pencil is 20 cm from the pole. The length of the image will be:
- (a) 5 cm
- (b) 15 cm
- (c) 30 cm
- (d) 6 cm
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3. If the focal length of a concave mirror is equal to that of a convex lens with radii of curvature 20 cm each, the radius of curvature of the concave mirror is:
- (a) 50 cm
- (b) 60 cm
- (c) 30 cm
- (d) 40 cm
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4. Two identical plano-concave lenses of radius of curvature 20 cm are placed with their curved surfaces in contact. If the empty space is filled with water (refractive index = 1.33), the focal length of the combination will be (R.I. of glass = 1.5):
- (a) 50 cm
- (b) 70 cm
- (c) 75 cm
- (d) 80 cm
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5. A person with farsightedness cannot see distinctly within 1 m. The power of lens used to see an object at a distance of 25 cm is:
- (a) 2.5 D
- (b) 4 D
- (c) 3.5 D
- (d) 3 D
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6. A convex lens of focal length 2 cm and refractive index 1.5 is used to see an object at a distance of 10 cm. The image formed will be at a distance:
- (a) -2.44 cm
- (b) -1.4 cm
- (c) -1.66 cm
- (d) 1 cm
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7. An object placed at a distance of 3 m forms a sharp image at 10 cm behind the lens. If the height of the image is 2 cm, the height of the object is:
- (a) 0.50 cm
- (b) 0.40 cm
- (c) 0.30 cm
- (d) 0.6 cm
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8. The compound microscope has a tube length of 20 cm. If the focal length of the eyepiece is 5 cm, and the object distance from the objective lens is 2 cm, the magnifying power of the microscope for relaxed eye will be:
- (a) 37.5
- (b) 40
- (c) 30
- (d) 25
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9. Two mirrors at an angle produce 6 images of a point. The number of images formed when the angle is increased by the same degree is:
- (a) 5
- (b) 4
- (c) 3
- (d) 6
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10. In case of a concave mirror, if the object is placed at the center of curvature of the mirror, the image will be:
- (a) Real, erect, and equal in size
- (b) Virtual, inverted, and equal in size
- (c) Real, inverted, and diminished
- (d) Real, inverted, and equal in size
Assertion and Reasoning Questions
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Assertion: Spherical mirrors may form virtual images.
Reason: Spherical mirrors form virtual images if the object is between the center of curvature and focus.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: A spherical mirror can form a virtual image when the object is within the focal length. However, the reason given is incorrect because the virtual image is formed only when the object is between the focal point and the mirror, not the center of curvature and the focus.
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Assertion: For a virtual object, the image formed by the concave mirror is certainly real.
Reason: The image formed by a concave mirror is certainly virtual if the object is real.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: Both statements are true, but they do not explain each other. A concave mirror forms a real image for a virtual object, while a real object forms a virtual image only under certain conditions.
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Assertion: An object placed perpendicular to the principal axis of a mirror will form an erect image if the object is real but the image is virtual.
Reason: An object placed perpendicular to the principal axis of a mirror will form an erect image if the object is virtual but the image is real.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: Both statements are true individually, but they describe different scenarios and do not explain each other directly.
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Assertion: The focal length of an equiconvex lens of radius of curvature \( R \) made of material of refractive index \( \mu = 1.5 \), is \( R \).
Reason: The focal length of the lens will be \( 2R \).- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (c)
Explanation: The assertion is correct for an equiconvex lens with \( R \) and \( \mu = 1.5 \), but the reason is incorrect because the focal length is not necessarily \( 2R \).
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Assertion: If the rays are converging after emerging from a lens, the lens must be convex.
Reason: The convex lens can give diverging rays.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (c)
Explanation: The assertion is correct as converging rays indicate a convex lens, but the reason is false as convex lenses do not diverge rays.
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Assertion: An objective lens with larger aperture allows higher resolving power for a telescope.
Reason: Objective lens of a telescope with a large diameter collects more light.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (a)
Explanation: Both the assertion and reason are correct, and the reason explains that more light collected by a larger aperture increases resolving power.
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Assertion: The power of a lens is more if the focal length of the lens is smaller.
Reason: A lens of larger focal length is used as an objective in a telescope.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: Although both statements are true, the reason does not explain the assertion directly.
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Assertion: The image of a point object situated at the center of curvature of a spherical mirror is formed at the center of curvature.
Reason: When the incident ray is parallel to the principal axis, the reflected rays pass through the focus of the concave mirror.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: The assertion is true, but the reason does not relate to the assertion as it discusses a different behavior of concave mirrors.
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Assertion: The total internal reflection occurs at the interface of the two optically different media.
Reason: The angle of incidence is greater than the critical angle in the denser medium, which is the necessary condition for total internal reflection.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (a)
Explanation: Both the assertion and reason are correct, and the reason explains the condition for total internal reflection.
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Assertion: When a ray of light enters from a denser medium to a rarer medium, it bends away from the normal.
Reason: As the angle of incidence increases, so does the angle of refraction.- (a) Assertion and Reason both are true and Reason is correct explanation of assertion
- (b) Assertion and Reason both are true but Reason is not correct explanation of assertion
- (c) Assertion is true but reason is false
- (d) Both Assertion and reason are false
Answer: (b)
Explanation: Both statements are true individually, but the reason does not explain why the ray bends away from the normal.
Case Study Questions
Case Study 1
A telescope is an instrument used for observing distant objects. It consists of an objective and an eyepiece. The objective has a larger focal length and aperture than the eyepiece. The magnifying power of the telescope is the ratio of the angle subtended at the eye by the final image to the angle subtended by the object at the lens.
Consider a telescope having an objective of focal length 144 cm and an eyepiece of focal length 6 cm.
Questions:
- 1.1 What is the magnification produced by the telescope?
- (a) 24
- (b) 30
- (c) 35
- (d) 40
- 1.2 A pair of objects is observed using the above telescope. If the actual separation between the stars is 6’, what will be their apparent separation observed through a telescope?
- (a) 60’
- (b) 144’
- (c) 200’
- (d) 30’
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