Refraction’ is the change in a waves
pathway due to a change in density, which affects the waves speed 1. The amount of refraction is determined by the angle
which the wave hits the boundary of the medium and the change in density 2.
Experiment A allows observation of the phenomenon – as the change in the lights
pathway through the transparent glass/perspex block, which is traced. A
numerical value is assigned to how the density affects the wave, which is
referred to as a ‘refractive index’. The refractive index is a value which
indexes how the speed of a light changes as it enters the material compared to
the speed of light in a vacuum 3. The refractive index of a material can be
experimentally found by measuring the angle of incidence and the angle of
refraction, and then substituting them both into Snell’s Law, (1). We can later compare our experimentally values to
the designated refractive indexes where: nair is 1.00 4, nglass
is 1.52 5,
Refractiveindex.info, 2018 and nperspex
is 1.50 6. Total internal reflection occurs when a wave hits a
boundary at an angle which is greater than the critical angle 7. To measure the critical angle in which ‘TIR’ can
occur, the equation (2).
With an ‘object’ pin and an ‘image finder’ pin on
either side of a convex lens, the focal length of the lens can be determined.
By nature, a convex lens creates an image which is real and upside down if
object is further from the lens than the focal point 8. The independent variable, the object pin, can be
changed to adjust ‘u’ – the distance between the lens and the ‘object’ pin.
Following the parallax method, when the ‘image finder’ pin is adjusted so that
the principle rays can be applied (as shown in diagram 1). Then, from
eye-level. through the focal lens, the two pins will look as though the tips
are just touching (shown in diagram 2). Using the lens equation; (3) 9, the equation (4) can be derived. Following this equation, if a
graph of uv against (u+v) is plotted, the gradient is equivalent to the focal
length. When plotted, the focal length can mathematically be found using; (5)