Class 12 Physics Electromagnetic Waves Visible or Light rays

Visible or Light rays

• Light waves are the most common form of EM wave.
• Their wavelength range is 4x1014 Hz-7x1014
• We are able to see everything because of light rays.
• The radiation which we get from sun is in the form of visible light.

• Most of the insects have compound eyes due to which they see not only the visible light but also the ultraviolet rays.
• Snakes can even see the infrared rays.

Problem:- About 5% of the power of a 100 W light bulb is converted to visibleradiation. What is the average intensity of visible radiation:-

(a) at a distance of 1m from the bulb? at a distance of 10 m?

Answer:- Power rating of bulb, P = 100 W

It is given that about 5% of its power is converted into visible radiation.

P’=(5/100)x100 =5W.

Hence, the power of visible radiation is 5W.

(a) Distance of a point from the bulb, d = 1 m

Hence, intensity of radiation at that point is given as:

I=(P’/4xπ xd2)

= (5)/ (4xπx1x1) =0.398W/m2

(b) Distance of a point from the bulb, d1 = 10 m

Hence, intensity of radiation at that point is given as:

I= (P’)/ (4xπ x(d1)2) =5/(4x x100x100)

=0.00398W/m2

.