Light and the electromagnetic spectrum Revision Notes

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.

Defining Lens Power

Lens power (P) quantifies how strongly a lens converges or diverges light. A higher power value means greater bending of light. It's measured in dioptres (D).

Relationship with Focal Length

Power is inversely proportional to focal length (f). The formula is P = 1/f. For this formula, focal length 'f' must be in metres (m) to yield power 'P' in dioptres (D). A shorter focal length corresponds to a more powerful lens.

Lens Shape and Power

Convex lenses converge light, are thicker at the centre, and have positive power (+D) with a real focal length. Concave lenses diverge light, are thinner at the centre, and have negative power (-D) with a virtual focal length. The degree of curvature of the lens surfaces directly influences its focal length and, consequently, its power.

Converging Lenses (Convex Lenses)

Converging lenses are thicker at the centre than at the edges. They refract parallel rays of light to meet at a single point called the principal focus (F) on the opposite side of the lens. This action is used to form real images, like in cameras and projectors, or magnified virtual images, such as in magnifying glasses. Essential rays for diagrams include: a ray parallel to the principal axis passes through F after refraction; a ray passing through the optical centre continues undeviated; and a ray passing through F before the lens emerges parallel to the principal axis.

Diverging Lenses (Concave Lenses)

Diverging lenses are thinner at the centre than at the edges. They cause parallel rays of light to spread out after refraction, as if originating from a virtual principal focus (F) located on the same side of the lens as the incident light. Diverging lenses always form virtual, upright, and diminished images. Key rays for diagrams include: a ray parallel to the principal axis appears to diverge from F after refraction; a ray passing through the optical centre continues undeviated; and a ray heading towards F on the opposite side of the lens emerges parallel to the principal axis.

Similarities and Differences in Ray Diagrams

Both lens types share common features in their ray diagrams, such as a principal axis, an optical centre, and focal points (F) and 2F points on both sides. Rays passing through the optical centre are undeviated in both. The main difference lies in how parallel incident rays behave: converging lenses focus them to a real focal point, while diverging lenses spread them out as if from a virtual focal point. This leads to converging lenses forming both real and virtual images, whereas diverging lenses only form virtual images. When drawing, remember to use arrows to indicate the direction of light and clearly label the type of lens and its focal points.

Transverse Nature of Electromagnetic Waves

All electromagnetic waves are entirely transverse. This means that the vibrations of the wave are at right angles (perpendicular) to the direction of energy transfer. Unlike longitudinal waves (e.g., sound waves), electromagnetic waves do not require a medium to travel and can travel through a vacuum.

Constant Speed in a Vacuum

A critical property of all electromagnetic waves, regardless of their frequency or wavelength, is that they travel at the same constant speed when moving through a vacuum. This universal speed is denoted as 'c' and is approximately 3.0 x 10^8 m/s. This speed remains consistent across the entire electromagnetic spectrum, from radio waves to gamma rays.