Light and EM Waves: Absorption and Diffuse Reflection

Basics of Absorption and Diffuse Reflection

• Light behaves in three ways when it encounters a material: it can be transmitted, absorbed or reflected.
• Absorption occurs when light energy is taken in by a material, often heating it up.
• Diffuse reflection, meanwhile, happens when light is scattered in all directions instead of being reflected in one direction, as in specular reflection.

Absorption

• Quite simply, absorption is about turning light energy into heat: when light hits a material, some or all of the light's energy is taken in by the material's atoms, which respond by moving faster and increasing the overall temperature.
• The darker the material, the more light it absorbs: black materials reflect very little light and absorb a lot, while white or shiny materials absorb less and reflect more.
• In the case of colour absorption, different materials absorb certain colours (wavelengths) of light while reflecting others, leading to the colour we observe: a green object absorbs all colours of light except green, which it reflects, and so appears green.

Diffuse Reflection

• Diffuse reflection doesn't follow the 'angle of incidence equals angle of reflection' rule, making objects look less polished or shiny.
• It occurs on rough surfaces, where the microscopic hills and valleys of the material scatter incoming light rays in all directions: a sheet of paper represents a rough surface, hence the reason we can see it from many angles.
• Clear images cannot form from diffuse reflection, as light rays scatter too much to create a discernible image.

Real-World Implications

• Absorption explains why certain materials warm up in sunlight while others remain comparatively cool.
• Understanding absorption is key to effective passive solar design, where buildings are designed to absorb sunlight and use it as natural, ecological heating.
• Diffuse reflection, meanwhile, is key to good illumination: soft, spread-out lighting is usually more desirable than harsh, concentrated light. It's why we value matte paints in photography or prefer indirect lighting in interiors to limit shadow formation.

Absorption of Electromagnetic Waves

• Like light, all electromagnetic waves can be absorbed, only more so with high-frequency waves like gamma rays and X-rays.
• It is necessary to shield against harmful high-frequency waves using suitable materials, such as lead aprons in hospitals for X-ray radiography to prevent overexposure.
• Lower-frequency waves, such as radio waves or microwaves, are more capable of passing through materials, but they can still be absorbed and pose safety issues, as in the case of microwave radiation causing burns.