Question

An object looks red when seen through a piece of red glass. What is the actual colour of the object ?
A. Red
B. White
C. Red or green
D. Black

Answer 1

Hint: When a light of only one color(monochromatic light) is passed into prism that light suffers deviation. We can relate that prism as a sudden medium changing instrument in the dispersion case. But when white light is passed the case is different. All this is due to change of medium which causes change in refractive index. We will answer this question by using these hints.

Complete answer:
When white light is passed through any transparent material let us assume prism the light gets separated into its individual component colors and gets out of that prism. This process of separation of light into its individual components is called dispersion of light
normally those colors are called as VIBGYOR where
VIOLET(V)
INDIGO(I)
BLUE(B)
GREEN(G)
YELLOW(Y)
ORANGE(O)
RED(R)
Each color has its own wavelength where violet has the shortest wavelength while red has the longest wavelength
Refractive index of each individual light varies with its wavelength and frequency. For higher frequency refractive index will be more and that color deviates more and lower frequency rays have less refractive index and deviates less

Now coming to the question, both prism and slab are made of glass. When we see through glass slab there will be no variation in medium. Hence refractive index won't change. Hence color won't change and actual color appears. Since the object is appearing red, its color is red. Here the color of the slab doesn’t matter.

So option A is correct.

Note:
If an object is red in color, it means that it absorbs all other colors and reflects red color. Hence its color is red. It is to be remembered that prism slows down violet light a lot and hence it will be travelling a longer path than red light inside prism. In the process of formation of rainbow raindrops act as prisms and cause dispersion of the sunlight. Scattering of light in fog is also due to dispersion of light by fog .