P5g - Refraction of waves Refraction is…

Refraction

When waves slow down, the wavelength gets shorter When waves speed up, the wavelength gets longer This causes the wave to change direction unless it is traveling along the normal.

Refraction investigation Intro You are going to calculate the refractive indices and critical angles for glass and perspex blocks

Refraction investigation Method 1.Watch the brief intro 2.Draw up 2 results tables, one for glass and one for perspex 3.Collect your apparatus 4.Repeat each experiment at least 5 times using the same angle i.

Refractive index (n) n = speed of light in a vacuum speed of light in the medium n = m/s speed of light in the medium

Refractive index (n) MediumSpeed of light (m/s) Vacuum Air Water n = speed of light in a vacuum speed of light in the medium Q ~ What is n for a) air b) water

Snell’s law i r n = sin i sin r medium i medium r “Angle of refraction depends on the refractive index”

Snell’s law n = sin i sin r Q ~ If the medium is water (n = 1.33) and angle i is 30 , what is angle r ? n = sin i sin r 1.33 = sin 30 sin r sin r = sin sin r = 0.5 = r = 22 

Snell’s law n = sin i sin r Q ~ In another medium, if i is 30  and r is 12 , what is n? Q ~ If the medium is water (n = 1.33) and angle i is 40 , what is angle r ?

Refraction investigation Analysis: Calculate n for air/glass and air/perspex interfaces using your experimental results 1.Pick one result from each material and use angles i and r 2.Use Snell’s law (n = sin i / sin r) to work out your values for n.

Refractive index between 2 media i r medium i medium r n = n r n i

Refractive index between 2 media i r water (n = 1.33) air (n = 1.0) n = n air = 1.0 n water 1.33 = 0.75 n = n r n i

Refractive index between 2 media i r water (n = 1.33) glass (n = 1.5) n = n glass = 1.5 n water 1.33 = 1.12 n = n r n i

Refraction investigation Analysis You can now calculate the refractive indices (n) for glass and perspex 1.Use your results n air/glass and n air/perspex 2.Assume that n air is 1.0

Refraction investigation i r air (n = 1.0) glass (n = ?) n = n glass = n glass n air 1.0 n = n r n i n glass = ?

Refraction investigation i r air (n = 1.0) perspex (n = ?) n = n perspex = n perpex n air 1.0 n = n r n i n perspex = ?

Refractive index and critical angle “Total internal reflection”

Refractive index and critical angle n = n r n i = sin i sin r = sin c sin 90 n r n i = sin c n r n i sin 90 = 1

Refractive index and critical angle = sin c n r n i Refraction investigation Now, using your results you can calculate: 1)The critical angle for glass 2)The critical angle for perspex We will check your answers with a practical next lesson