Q 01/05

How does an incandescent light bulb produce light?

A) By exciting mercury vapour with electricity

B) By passing current through a semiconductor

C) By burning a gas inside the glass

D) By heating a filament until it glows

Answer · why

D) By heating a filament until it glows

The filament sits in a bulb that is either evacuated or filled with inert gas to stop it oxidising.

Q 02/05

Roughly what share of the energy consumed by an incandescent bulb becomes visible light?

A) About 20 percent

B) Less than 5 percent

C) About 40 percent

D) About 65 percent

Answer · why

B) Less than 5 percent

The rest is released as heat, which is why they work as heat lamps for incubators and Easy-Bake Ovens.

Q 03/05

What is the luminous efficacy of a 120-volt incandescent bulb?

A) 16 lm/W

B) 4 lm/W

C) 40 lm/W

D) 60 lm/W

Answer · why

A) 16 lm/W

A compact fluorescent manages about 60, which is why governments have banned the old bulbs to save energy.

Q 04/05

In 1802 Humphry Davy made incandescent light by passing current through a strip of which high-melting metal?

A) Copper

B) Iron

C) Silver

D) Platinum

Answer · why

D) Platinum

He used a battery of 2,000 cells in the basement of the Royal Institution; it was too dim and short-lived to be practical.

Q 05/05

What was the first practical electric light, invented by Davy in the first decade of the 1800s?

A) The carbon arc lamp

B) The gas mantle

C) The neon tube

D) The mercury-vapour lamp

Answer · why

A) The carbon arc lamp

Arc lamps burned their carbon rods fast and gave out kilowatts of light, so they suited only large spaces.