4.3 Wave Intensity - This is defined as the amount of
energy per unit time flowing through unit area (It is normally measured
in W.m-2)
If light from a point source spreads out spherically, the intensity at any distance can be found by dividing
the power by the surface area of a sphere of radius equal to the distance.
The intensity of a wave is proportional to the square of its
amplitude
I α a2
(or I = ka2)
1. A wave of amplitude a has an intensity of 3.0Wm–2.
What is the intensity of a wave of the same frequency
that has an amplitude 2a?
2. A wave has an amplitude of 0.02 m and an intensity of
2.5 Wm-2. It’s amplitude is increased until its intensity
-2
reaches 5 Wm . What is the new amplitude?
3. A light bulb gives out visible light energy at a rate of 60W. What is the intensity at a
distance of 5 m from the light bulb (imagine the light spread over a sphere of radius 5m).
4. The sun’s total power output is 3.9 x 1026 W (known as its “luminosity”). What is the
intensity of sunlight at the surface of the earth’s atmosphere?
(The earth orbits at an average distance of 1.5 x 1011 m from
the sun.
5. What power would a light bulb have to have to produce the
same intensity as you calculated in question 4 if the light bulb
were 3 m away from you.
6. Proxima Centauri (the closest star to the earth after the sun) has a total power output
(luminosity) of 6.6 x 1023 W. The distance of Proxima Centauri from the earth is 4.2 light
years. Calculate the intensity of light from Proxima Centauri that reaches the earth’s
upper atmosphere.