A light bulb emits light uniformly in all directions. the intensity of the light

Question

A light bulb emits light uniformly in all directions. the intensity of the light 3m from the bulb is 0.8 W/m^2.

What is the power (energy per second) that the light bulb emits?

A real object is 1m in front of a lens. Then lens forms an upright image of the object, that is 1/2 the objects height, ie.

the absolute value of the image height is 1/2 the absolute value of the object height.

what is the focal length of the lens, and what type of lens is it ( concave or convex)?

please, show work and formulas for both of the questions to get the points, thank you !

Explanation / Answer

1)

intentsity = power/area

power = intensity*area

= 0.8*4*pi*3^2

= 90.432 watts

2)
u = 1m

magnification, m = y'/y

m = 1/2

m = -v/u

1/2 = -v/u
v = -u/2


1/f = 1/v + 1/u

1/f = -2/u + 1/u

1/f = -1/u

f = - u = -1 m

it is a concave lense

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