1 reflection and refraction
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Transcript of 1 reflection and refraction
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11 Reflection and Refraction
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Introduction
To explain how light behaves, we can think of light travelling as rays.A ray travels in a straight line. It will change direction if:• It is reflected (when it strikes a surface)• It is refracted (when it passes from one material
to another).We can also think of light as wavesLaws of reflectionThe laws of reflection tells us where a ray will go when it is reflected.The normal is the line at 90o to the reflecting surface at the point where the incident ray strikes it
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normalincident ray reflected rayi r
Law 1: angle of incidence = angle of reflection, i = r (angles measured from the normal to the ray)
Law 2: incident ray, reflected ray and the normal are all in the same plane.
These laws not only apply for flat surfaces but also for curved and rough surfaces
normal
i r ir
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Refraction: when it happensLight travels fastest in a vacuum. It travels more slowly in other media.When light changes speed (because it travels from one medium to another), it is refracted.• If a ray enters a medium head-on (angle of
incidence i = 0), it travels straight on.• If a ray enters a medium obliquely, it bends
Laws of refractionAs with reflection, angles are measured from the normal to the ray.Law 1: Snell’s law explains how the angles of incidence and refraction are related.Law 2: incident ray, refracted ray and the normal are all in the same plane.
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normalr r
i i
Note that r is now the angle of refraction, not reflectionRefractive index nThe refractive index, n of a medium relates the speed of light in the medium to the speed of light in free space (vacuum)
Refractive index, n =
speed of light in free space speed of light in medium
n =
c0
cmedium
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Refractive index nIn a medium of refractive index 2, light travels at half its speed in free space (vacuum). Some values of n are worth remembering:n0 = 1 (by definition)
nair = 1.00 (to 2 decimal places)
nwater = 1.33
nglass ~ 1.5 (depending on the composition of glass)
Snell’s lawFor a ray passing from air into a medium of refractive index n, the angle of incidence i and the angle of refraction r are related by:
n =sin isin r
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Snell’s lawWhen a ray passes from one medium to another, the refractive index can be calculated using the equations:
n =
ci
cr
=nr
ni
Example
A ray of light travels from glass (ni = 1.5) into water (nr = 1.33) with an angle of incidence i of 30o. Calculate the angle of refraction r.Step 1: calculate the relative refractive index from the values for the two materials:
n =
nr
ni
=1.33 1.5
= 0.887
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Step 2: Substitute values into the Snell’s law equation, rearrange and solve:
n = sin isin r
so 0.887 =
sin 30o
sin r
sin r =
sin 30o
0.887= 0.504
so r = 34o
Questions:
1. Does a ray speed up or slow down when it enters a more dense medium?
2. If the angle between the incident ray and the reflective surface of a mirror is 35o, what are the angles of incidence and refraction?
3. A ray of light, travelling through air, strikes a glass surface with an angle of incidence of 40o. The refractive index of glass is 1.47. draw a diagram to show the situation. Calculate the angle of refraction.