CBSE X(10th) Human Eyes , Reflection and Refraction (Solution)

NCERT Solution, MCQs, Study Notes, Q & A

Question: What is the minimum distance between an object and its real image in case of a concave mirror?

Ans: The object and the image, both are at the centre of curvature of the concave mirror. The image formed is real and inverted.  The minimum distance  between object and its image =  0 units

Question:  Why is the refractive index of atmosphere different at different altitudes?

Ans: it is because it is a mixture of differnet particles and gas molecules which often has diff. refractive indices

Question:  When a light ray passes obliquely through the atmosphere in an upward direction, how does its path generally change?

Ans: When a light ray passes obliquely through the atmosphere in an upward direction, it 's path changes due to atmospheric refraction. This refraction happens because each atmospheric layer has a different optical density, due to which its path changes.

Question:  State two factors on which the lateral displacement of the emergent ray in a rectangular glass slab depends for an oblique incident ray on the opposite face

Ans: The thickness of the glass slab and the optical density of the glass slab affect the lateral displacement of the emergent ray.

Question:  Between which two points related to a concave mirror should an object be placed to obtain on a screen an image twice the size of the object?

Ans : Between center of curvature and Principal Focus

Question:  What is meant by ‘total internal reflection’? State two essential conditions for total internal reflection to take place. With the help of a ray diagram, illustrate an application of total internal reflection.
Ans: 

When light passes from one medium to other, it bends from its path which is called refraction. When a light ray passes from denser to rarer medium, at some angle of incidence it does not go out of the denser medium but just grazes along the boundary (i.e. angle of refraction = 90⁰). This angle of incidence is called critical angle.

Now, if you increase this angle of incidence, light ray is reflected back into the denser medium, which is called total internal reflection. The light ray is reflected every time it hits the boundary because angle if incidence is equal to angle of reflection, and is not able to come out of the medium. This phenomenon is called total internal reflection.

The conditions for total internal reflection are:
 (i) the light ray must be travelling from optically denser medium to rarer medium and 
(ii) the angle of incidence must be greater than the critical angle.

Optical fiber communication uses the principle of total internal reflection for transferring data through optical fibers.

Question:  All the images formed by two plane mirrors inclined at an angle with respect to each other lie in a   a) circle b) straight line c) sphere d) none of these

Ans: All the images formed by two plane mirrors inclined at an angle with respect to each other lie in a straight line. It happens in the barber shop.(Answer by JSUNIL)

Q. The image formed by a convex mirror of focal length 30cm is a quater of the object . what is the distance of the object from the mirror ?

Ans: h_i / h_o = 1/4

h_i = image height

h_o = object height

now as

h_i / h_o = -v/u = 1/4

we get

v = -u/4

so, using this in eq. (1), we get

1/30 = -4/u + 1/u

or

1/30 = -3/u

thus, the object distance will be

u = -90 cm

Q. A concave mirror produces 10cm long image of an object of height 2 cm . what is the magnification produced ?

Ans: The magnification of a curved mirror is given as

m = h_i / h_o

here

h_i = height if the image = 10cm

h_o = height of the object = 2cm

so, we have

m = 10cm / 2cm

so magnification will be m = 5 

Question:  If a fish inside a pond is viewing a man outside the pond then, how will the image of the man appear?

Ans: Due to the refraction of light rays, to the fish, the man would appear to be higher than he actually is.

Question: A man sits in a optician 's chair ,looking into a plane mirror which is 2m away from him and views the image of a chart which faces the mirror and is 50 cm behind his head.How far away from his eyes does the chart appears to be.

Ans: The distance between the eye and chart will be the sum of the distance between eye and the mirror and the miror and chart image. So we will get  In the mirror the chart appears to be 2m + 2m + 0.5m = 4.5m away.

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