Physics 7.2 (Light and Optics)

Defraction Light changes direction when passing through a glass prism due to different speeds of wavelengths (light slows as it goes through dense materials)

Short wavelengths eg. violet change direction more because they slow down more

Lens with a slightly bent face which bends light to a focal point

Thick: short focal length, more powerful Thin: long focal length, less powerful

Focal length: distance from centre of the lens to focal point Focal point: where the parallel rays of light meet when passing through a lens 

Power (D) = 1 / Focal length

Rays of light that pass through the centre continue straight on. All rays converge at the focal point

Lenses are heavy, which makes them difficult to move

It is also difficult to make lenses uniform

Lenses absorb some of the light radiation

Lenses produce chromatic aberration (different colours have different focal lengths)

Lenses only work well for visible light

Eyepiece lens is more powerful (thicker) than the objective lens

Magnification = F_o / F_e 

Most telescopes use a concave mirror

Aperture: a hole in optics through which radiation travels (a gap)

Narrow gap/longer wavelength makes a longer wavelength

When: gap < wavelength, max. defraction ; gap = wavelength, little defraction ; gap > wavelength, no defraction

High levels of defraction produces blurry images and so telescopes should have as large an aperture as possible to produce sharp images

Diffraction grating: set of narrow evenly spaced parallel lines on a thin sheet of glass. Different colours emerge at different angles to produce several spectra