If you fire a bullet through a board, it will slow down inside and emerge at a speed that is less than the speed at which it entered. Does light, then, similarly slow down when it passes through glass and also emerge at a lower speed?

Explanation:

In case of bullet the Kinetic energy = 1/2 m v²

here m is the mass of bullet and v is its velocity .

When it penetrates through the board , the velocity of bullet decreases due to frictional force of board acting in opposite direction .

But in case of light photon K.E = h ν

here h is the Planck's constant and ν is the frequency of light wave .

When it passes through the glass , The wavelength λ of light decreases inside the glass .

The velocity of light c = ν λ , Because the frequency ν is the property of source , so it cannot be changed by the medium i.e glass .

Thus with the decrease of wavelength , the velocity decreases .

When light wave moves out of the glass , its wavelength again becomes the same . The velocity increases to its previous value .

That is the reason by which the light is emitted with the same velocity .

5 w for 1hr = 5x60x60 joules=300x60=3000x6=18,000j as total energy conversion. efficiency of bulb needed for radiant to thermal within 18000

see earlier answer