Optics and Light




With the recent publication of PHYSICS IS... there are now three Ask the Physicist books! Click on the book images below for information on the content of the books and for information on ordering.


QUESTION:
Why we cannot feel the momentum or the force of the sunlight?

ANSWER:
The most intense light from the sun is about λ=550 nm=5.5x10-7 m (yellow); the corresponding frequency is f=c/λ=3x108/5.5x10-7=5.45x1014 Hz. The momentum of one photon is p=hf/c=6.6x10-34x5.45x1014/3x108=1.2x10-27 kg·m/s. The photon flux (photons per second per square meter) for photons of this wavelength is about
Φ=3x1021 s-1·m-2. So if I estimate your area to be about 1 m2, there are about 3x1021 photons hitting you every second. The total momentum imparted to you over that second (assuming that you absorb the photons) is about Δp=3x1021x1.2x10-27=3.6x10-6 kg·m/s. The average force you feel is about Fpt=3.6x10-6 N; and this force is spread uniformly over your area. This is really small; for example, if we take the typical weight of a one dust particle to be about 10-8 N, the weight of about 360 dust particles spread over your whole body is about what you "feel" from the sunlight.


QUESTION:
Assume there are 2 black bodies, 1 with a mass double of the other. If both black bodies reach a temperature of 5000K, do both emit blue light as black body radiation, with the heavier body emitting a brighter light. Or does the heavier body emit white light while the lighter body emits blue light? Why?

ANSWER:
A black body does not radiate one color but rather a spectrum of colors. The spectrum for 5000 K is shown above. The most intense color of this spectrum is at 580 nm which is the green color shown to the right. The intensity of the light emitted does not depend on the mass, it depends on the surface area of the black body; if you double surface area, you double the amount of energy being radiated.


QUESTION:
The demonstration wherein one pours water from a clear vessel and one shines a laser pointer (a red laser pointer in my case) through the water and it appears the laser beam bends with the water pouring out of the clear vessel; What causes the beam to bend with the water? Would this same type of effect work with other fluids than water, such as air or glass?

ANSWER:
It is caused by total internal reflection. If the angle which the light hits the surface between the water and the air is glancing enough, the light will be reflected back into the water rather than be refracted into the air. Total internal reflection can happen whenever light strikes the interface with a material of smaller index of refraction; the minimum angle for which it can occur depends on the relative indices of refraction.