Light and the electromagnetic spectrum Study Guide

Lenses are optical devices used to refract light and form images. There are two primary types: converging (convex) and diverging (concave). A converging lens is thicker in the middle and causes parallel light rays to converge to a focal point. A diverging lens is thinner in the middle and causes parallel light rays to spread out, appearing to originate from a virtual focal point. The type of image formed depends on the lens type and the object's position relative to the lens's focal point. Real images are formed when actual light rays converge; they can be projected onto a screen and are always inverted. Virtual images are formed when light rays appear to diverge from a point; they cannot be projected and are always upright. Converging lenses can form both real and virtual images depending on object distance, while diverging lenses always form virtual, upright, and diminished images.

The perceived colour of an opaque object is determined by the wavelengths of light that it reflects. When white light, which contains all visible colours, strikes an object, some wavelengths are absorbed, and others are reflected. The colour we observe is that of the reflected wavelengths. For example, a red apple appears red because it absorbs most wavelengths of visible light but reflects red light. Similarly, a green leaf absorbs most colours but reflects green light. Black objects absorb nearly all wavelengths of light and reflect very little, hence appearing black. White objects reflect nearly all wavelengths and absorb very little, making them appear white.

Filters work differently. They selectively transmit certain wavelengths of light while absorbing most others. A red filter, for instance, allows only red light to pass through it, absorbing all other colours. If white light passes through a red filter, only red light emerges. If green light is shone through a red filter, no light will pass through, as the green light will be absorbed by the red filter. The final colour perceived after light passes through a filter is determined by the colours present in the incident light and the specific wavelengths that the filter is designed to transmit.