Click image to open full size
Light is a form of energy that lets us see the world and powers many natural and human-made processes. The Sun is Earth's main natural source of light energy, while lamps and bulbs provide artificial light when sunlight is unavailable. Both sources send energy outward as electromagnetic waves, especially in the visible range that human eyes can detect. Understanding light energy helps explain vision, heating, electricity use, and solar power.
The Sun produces light through nuclear fusion, releasing enormous amounts of energy that travel through space to Earth. Lamps produce light by converting electrical energy into light energy, although some of the energy also becomes heat. When light reaches an object, it can be absorbed, reflected, or transmitted depending on the material. The brightness we observe depends on the source, the distance from it, and how much light enters the eye or falls on a surface.
Key Facts
- Light is electromagnetic energy that can travel through empty space.
- Visible light is the part of the electromagnetic spectrum detected by the human eye.
- The speed of light in vacuum is c = 3.0 x 10^8 m/s.
- Light energy of one photon is E = hf.
- Wavelength and frequency are related by c = fλ.
- Light intensity decreases with distance according to I ∝ 1/r^2.
Vocabulary
- Light energy
- Light energy is energy carried by electromagnetic waves that can travel from a source to other places.
- Visible light
- Visible light is the small range of electromagnetic waves that human eyes can detect.
- Intensity
- Intensity is the amount of light energy passing through a certain area each second.
- Reflection
- Reflection is the bouncing of light off a surface.
- Absorption
- Absorption is the process in which a material takes in light energy, often converting it to heat.
Common Mistakes to Avoid
- Thinking light from a lamp and light from the Sun are different kinds of energy, when both are electromagnetic radiation and both can include visible light. The source is different, but the basic type of energy is the same.
- Assuming all electrical energy in a bulb becomes light, which is wrong because real bulbs also produce heat. This is why some bulbs feel hot and have less than 100 percent efficiency.
- Believing brighter means the source always has more energy output, which is wrong because distance strongly affects how bright a source appears. A weaker nearby lamp can look brighter than a stronger distant source.
- Confusing reflection with transmission, which leads to wrong predictions about what happens when light hits glass, mirrors, or dark surfaces. You must identify whether light bounces, passes through, or is absorbed.
Practice Questions
- 1 A photon of light has frequency 6.0 x 10^14 Hz. Using E = hf with h = 6.63 x 10^-34 J s, calculate the energy of one photon.
- 2 A lamp gives an intensity of 120 W/m^2 at 2 m. If the distance is increased to 4 m, what is the new intensity assuming I ∝ 1/r^2?
- 3 A black shirt and a white shirt are placed in sunlight for the same amount of time. Explain which one usually becomes warmer and why in terms of absorption and reflection of light energy.