Light and the electromagnetic spectrum Revision Notes

Specular Reflection

Occurs on smooth, polished surfaces (e.g., mirrors). Incident parallel light rays reflect as parallel rays in one specific direction. This results in the formation of a clear, sharp, reflected image. The angle of incidence equals the angle of reflection for each ray.

Diffuse Reflection

Occurs on rough or uneven surfaces (e.g., brick walls, matte paper). Incident parallel light rays scatter in many different directions due to surface irregularities. This type of reflection does not form a clear image but makes objects visible from various viewing angles.

Law of Reflection

Both specular and diffuse reflection adhere to the law of reflection at a microscopic level. For any single light ray reflecting off a point on a surface, the angle of incidence is always equal to the angle of reflection, measured relative to the normal at that point.

Microwaves and Internal Heating

Microwave radiation, commonly used in microwave ovens and communication, can penetrate human tissue. When absorbed in sufficient quantities, this energy causes water molecules within cells to vibrate rapidly, leading to a rise in temperature. This internal temperature increase can damage or destroy cells and tissues, particularly in organs with high water content like the eyes (e.g., cataracts) or internal organs, without necessarily causing a sensation of external heat.

Infrared and Skin Burns

Infrared (IR) radiation is primarily associated with heat. When the skin is exposed to excessive levels of IR, the energy is absorbed, significantly increasing the skin's temperature. This can lead to thermal burns, similar to touching a hot surface. The immediate sensation of heat usually serves as a warning sign, prompting individuals to withdraw from the source, but prolonged or intense exposure can still cause severe damage.

Ultraviolet (UV) Radiation and Cell Damage

Ultraviolet (UV) radiation, a component of sunlight, has enough energy to cause damage to surface cells. This includes skin cells and cells in the eyes. Prolonged or intense exposure to UV can lead to DNA damage within skin cells, increasing the risk of skin cancer (e.g., melanoma). In the eyes, UV can contribute to conditions like cataracts and photokeratitis (corneal sunburn), impairing vision.

X-rays and Gamma Rays: Mutation and Cellular Damage

X-rays and gamma rays are high-energy forms of electromagnetic radiation. They are ionising radiation, meaning they carry enough energy to remove electrons from atoms, creating ions. This ionisation process can break chemical bonds within DNA molecules, leading to mutations or direct damage to cells. Such damage can kill cells, cause them to malfunction, or result in uncontrolled growth (cancer). This is why exposure to these radiations is strictly controlled, especially in medical imaging and industrial settings.

Reflection and the Law of Reflection

Reflection is when a light ray bounces off a surface. The 'normal' is a line perpendicular to the surface at the point of incidence. The law of reflection states that the angle of incidence is equal to the angle of reflection. Both angles are measured from the normal. Incident ray, reflected ray, and the normal all lie in the same plane.

Refraction and Total Internal Reflection (TIR)

Refraction is the bending of light as it passes from one medium to another due to a change in speed. Light bends towards the normal when entering a denser medium and away from the normal when entering a less dense medium. Total Internal Reflection (TIR) occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle, causing all light to reflect back into the denser medium.

Colour of Opaque Objects

The perceived colour of an opaque object depends on which wavelengths of light it reflects when illuminated by white light. White light contains all colours of the visible spectrum. Objects absorb some wavelengths and reflect others. The reflected wavelengths determine the object's colour. For example, a blue object absorbs all colours except blue, which it reflects.

Light Filters and Transmission

Filters function by selectively transmitting certain wavelengths of light. A coloured filter absorbs most colours while allowing its own colour to pass through. For example, a yellow filter transmits yellow light and absorbs other colours like blue. If light of a colour that the filter absorbs is incident on the filter, no light will pass through.

Black, White, and Primary Colours

Black objects absorb almost all incident light wavelengths and reflect very little, leading to their dark appearance. White objects reflect almost all incident light wavelengths, making them appear bright. The primary colours of light are red, green, and blue. Mixing them in different proportions can create other colours of light.