Chapter 9: Light - Reflection and Refraction

Multiple Choose Question answer :

Question:  Which one of the following materials cannot be used to make a lens?
(a) Water      (b) Glass     (c) Plastic    (d) Clay

Answer: (d) Clay

[ Lenses need transparent materials to refract light, but clay is opaque, so it cannot focus or bend light properly.]
Question:  The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature
(b) At the centre of curvature
(c) Beyond the centre of curvature
(d) Between the pole of the mirror and its principal focus.

[A concave mirror forms a virtual, erect, and magnified image only when the object is placed within the focal length (i.e., between the pole and the principal focus).]

Answer: (d) Between the pole of the mirror and its principal focus.
Question:  Where should an object be placed in front of a convex lens to get a real image of the size of the object?
(a) At the principal focus of the lens
(b) At twice the focal length
(c) At infinity
(d) Between the optical centre of the lens and its principal focus.

Answer: (b) At twice the focal length .

[When the object is placed at 2F, the convex lens forms a real, inverted image of the same size at 2F on the other side.]

Question: A spherical mirror and a thin spherical lens have each a focal length of –15 cm. The mirror and the lens are likely to be
(a) both concave.
(b) both convex.

(c) the mirror is concave and the lens is convex.
(d) the mirror is convex, but the lens is concave.

Answer:  (a) both concave.

[ Negative focal length indicates a concave mirror and a concave lens, as both converge light in front of the surface. ]

Question:  No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
(a) plane.
(b) concave.
(c) convex.
(d) either plane or convex.

Answer: (d) either plane or convex.

[ Both plane mirrors and convex mirrors always form erect images, regardless of the object’s distance.]
Question:  Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.

Answer: (c) A convex lens of focal length 5 cm. 

[A convex lens with short focal length acts as a magnifier, producing a larger, clearer image of small letters when held close to the eye.]

Short Question answer :

Question: Define the principal focus of a concave mirror. [1M]

Answer:  A number of rays parallel to the principal axis are falling on a concave mirror and the rays are intersecting at a point on the principal axis of the mirror. This point is called the principal focus of the concave mirror.
Question: The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Solution:  Here ,  

We have,

Therefore, the focal length is 10 cm .

Question: Name a mirror that can give an erect and enlarged image of an object. [1M]
Answer:  concave mirror . If the position of the object is between the pole and the focus , then the concave mirror gives an erect and enlarged image of an object .

Question: Why do we prefer a convex mirror as a rear-view mirror in vehicles ?

Answer:  Convex mirrors are preferred because they always give an erect, though diminished, image. Also, they have a wider field of view as they are curved outwards. [1M]

Question: A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located ? [2M]

Solution:  For concave mirror :  Here, and

We have,  

Therefore, the image is located within 30 cm .

Question: Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass ? The speed of light in vacuum is  .

Solution: Here, The refractive index ,  and  
We know that ,

 

 

Therefore, the speed of light in the glass is  .

Question: A convex mirror used for rear-view on an automobile has a radius of curvature of 3.00 m. If a bus is located at 5.00 m from this mirror, find the position, nature and size of the image. [3M]

Solution: Here,  , ,   and

Using mirror formula, 

     

 

 

 

Therefore, the position of the object is 1.15 m .

Magnification

The nature of the of the image is virtual and erect .  The size of the image is diminished .
Question: An object, 4.0 cm in size, is placed at 25.0 cm in front of a concave mirror of focal length 15.0 cm. At what distance from the mirror should a screen be placed in order to obtain a sharp image? Find the nature and the size of the image.  [3M]

Solution:  Here, ,  , ,  and

Using mirror formula, 

  

 

 

 

 

 

Magnification

The screen should be placed at 37.5 cm from the mirror.

and

The height of the image is 6 cm . The nature of the image is real and inverted . The size of the image is enlarged.

Long Question answer :

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