Topic 5: Light and the EM spectrum

Critical Angle:

The angle of incidence at 90 degrees. Exceeding the critical angle/ 90 degrees will cause total internal reflection as ligjt cannot pass through the surface

Constant Temperature:

Bodies at a fixed temperature emit the same average power that they absorb.

Concave Lens:

• Shape: Thicker at the edges, thinner in the center.

• Effect on Light: Parallel rays bend outward (diverge) after passing through.

• Why?

  • When a parallel ray enters the lens, it refracts at the first curved surface, bending toward the normal because glass is denser than air.

  • At the second surface (where the ray exits), it bends away from the normal.

  • Because the lens is thinner in the middle, the overall effect is that the rays spread apart (diverge).

Convex Lens:

A lens that is thicker in the centre than at the edges. Parallel rays converge when passing through.

How does the difference in lens shape lead to different outcomes for light rays?
When a parallel ray enters, it bends toward the normal at the first curved surface.

• At the second surface (where it exits), it bends away from the normal.

• Because the lens is thicker in the center, this causes the rays to bend inward (converge) toward a focal point in front of the lens.A lens that is thicker at the edges than in the center, causing parallel rays to diverge when passing through.

Diffuse Reflection:

The reflection of a wave from a rough surface that results in the wave being scattered.

Electromagnetic Spectrum:

A group of transverse waves that cover a large range of frequencies and wavelengths. The highest frequency waves in the spectrum are gamma-rays and the lowest are radio waves.

Electromagnetic Waves:

Transverse waves that transfer energy from the source of the waves, to an absorber. They form a continuous spectrum of different frequencies and all travel at the same speed in a vacuum.

Focal Length:

The distance between the centre of a lens and its principal focus.

Gamma Rays:

High energy radiation rays used for detecting and treating cancers, and sterilising food and medical implements. They can cause cell damage and mutations.

Infrared Radiation:

A type of radiation that all objects emit and absorb. The hotter an object is, the greater the infrared radiation it emits in a given time.

Infrared:

Used for cooking food, thermal imaging and short range communications.It can cause skin burns.

Law of Reflection:

The angle of incidence must always equal the angle of reflection when a wave reflects.

Lens:

An object that forms an image through the refraction of light.

Microwaves:

Used for satellite communications and for cooking food. They can cause internal heating of body cells.

Power of a Lens:

Equal to the reciprocal of the focal length of a lens. Concave lenses have negative powers whereas convex lenses have positive powers.

Principal Focus:

The point before or after a lens where light rays appear to meet. The rays may converge or diverge away from this point.

Radio Waves:

Used for television and radio signals. They can be produced by oscillations in electrical circuits, or can induce these oscillations themselves.

Ray Diagram:

A visual representation of the path of a wave, usually around the point where it meets a boundary. Rays are usually drawn as straight lines with an arrow pointing in their direction of travel.

Real Image:

An image produced by light-rays physically converging. Real images are ones that can be projected onto a screen.

Reflection:

The bouncing back of a wave at a boundary.

Refraction:

The changing of speed, and consequently the direction, of a wave as it changes medium. The wavelength of the wave will also change but the frequency remains constant.

Specular Reflection:

The reflection of a wave from a smooth surface.

Speed of EM Waves:

All electromagnetic waves travel at the same speed in a vacuum (3 x 10 m/s).

Total Internal Reflection:

A process that occurs when a wave meets the boundary from a medium with a high refractive index to one with a lower refractive index, at an angle of incidence greater than the critical angle. All the wave is reflected at the boundary.

Ultraviolet:

Used in energy efficient lamps, disinfecting water, and for sun tanning. It can cause cell and eye damage that can result in skin cancer and eye conditions.

Virtual Image:

An image produced by the apparent, but not actual, divergence of light-rays. Virtual images cannot be projected onto a screen.

Visible Light:

The only type of electromagnetic radiation that our eyes can detect. It is used for fibre optic communications and photography.

X-Rays:

Used for medical imaging and security scanners. They can cause cell damage and mutations.