Halogen bulbs have filaments like regular incandescent bulbs, but the filaments

Question

Halogen bulbs have filaments like regular incandescent bulbs, but the filaments are much hotter, the bulbs are physically smaller, and they work at lower voltages. The small bulbs can be used at the focal point of a mirror to create a tight beam of light; they are used for spotlights, projectors and desk lamps.

1) Increasing the temperature of a bulb’s filament ______________ the energy radiated from the filament, _____________ the peak wavelength of the emitted radiation.

A. increases, increases

B. increases, decreases

C. decreases, increases

D. decreases, decreases

2) Light from a halogen desk lamp is focused into a circle 40 cm across. A student using the lamp needs brighter light; she moves the lamp closer to the desktop to focus the light into a circle 20 cm across. This increases the intensity by a factor of

A. 2

B. 4

C. 6

D. 8

Explanation / Answer

1) (B) increases, decreases.

Increasing the temperature naturally increases the energy and by Wien displacement law, the peak wavelength is reduced due to increase in temperature.

2) (B) 4

Intensity inersely propoertional to area:

I -> 1/A

or, I -> 1/r2

So, I2/I1 = (r1/r2)2

=> I2 = (r1/r2)2 * I1

= (40/20)2 * I1

= 4 I1

Thus the intensity increases by a factor of 4.

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