# Important Questions for Class 12 Chapter 9 - Ray Optics and Optical Instruments

Important questions based on NCERT syllabus for Chapter 9 - Ray Optics and Optical Instruments:

Question-1: A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is
(a) blue
(b) green
(c) violet
(d) red

Solution:
We know that, when a light ray goes from one medium to other medium, the frequency of light remains constant.
We also know v ∝ λ,
Since, the wavelength of red coloured light is highest wavelength. Hence, it has highest speed.
Therefore, the red colour will emerge first after travelling through the slab.

Question-2: An object approaches a convergent lens from the left of the lens with a uniform speed 5 m/s and stops at the focus. The image
(a) moves away from the lens with an uniform speed 5 m/s
(b) moves away from the lens with an uniform acceleration
(c) moves away from the lens with a non-uniform acceleration
(d) moves towards the lens with a non-uniform acceleration

Solution:
If an object approaches a convergent lens from the left of the lens with a uniform speed of 5 m/s, then the image moves away from lens with a non-uniform acceleration.
A car is moving with at a constant speed of 60 km h-1 on a straight road. Looking at the rear view mirror, the driver finds that the car following him at a distance of 100 m and is approaching with a speed of 5 kmh-1.

Question-3: In order to keep track of the car in the rear, the driver begins to glance alternatively at the rear and side mirror of his car after every 2 s till the other car overtakes. If the two cars were maintaining their speeds, which of the following statement (s) is/are correct?
(a) The speed of the car in the rear is 65 km h-1
(b) In the side mirror, the car in the rear would appear to approach with a speed of 5 kmh-1 to the driver of the leading car
(c) In the rear view mirror, the speed of the approaching car would appear to decrease as the distance between the cars decreases
(d) In the side mirror, the speed of the approaching car would appear to increase as the distance between the cars decreases

Solution:

We know that, the image formed by convex mirror does not depend on the relative position of object wit mirror. Therefore, the speed of the approaching car would appear to increase as the distance between the cars decreases in the side mirror.