4.3 – Reflection
IGCSE Physics
Why do we see things?
We see things because light coming from
them enters our eyes.
Light comes from things in two ways:
• They are luminous (stars, bulbs,
screens, fire, glowworms)
• They reflect light (pretty much
everything else)
Law of Reflection
The angle of reflection equals the angle of
incidence
SHINY
NOT SHINY
CONVEX MIRRORS
Image always upright, behind the mirror (virtual)
and smaller
Mirror Equation
1
𝑓
=
1
𝑠𝑜
+
1
𝑠𝑖
ℎ𝑖
M=
ℎ𝑜
=−
f: + (converging lens or concave mirror)
f: - (diverging lens or convex mirror)
s: + (real image)
s: - (virtual image)
M: Magnification rate
hi: image height
ho: object height
so: object’s distance
si image’s distance
𝑠𝑖
𝑠𝑜
CONCAVE MIRRORS
More complicated. Depends where the
object is relative to the focal point.
However, for IGCSE, light FROM focal
point forms a beam (flashlights and
headlights), and light from afar is focused
at the focal point (satellite dishes and
solar heaters)
Is more light produced?
4.4 –
Refraction
IGCSE Physics
Refraction cause optical illusions and effects
When light meets a transparent medium it
bends (refracts)
It does this because it SLOWS down. The
effect is only noticeable if it strikes at an
angle to the normal.
The ratio of the change in speed is known
as the REFRACTIVE INDEX, 𝑛. The
refractive index of a vacuum (and
essentially air) is 1.0. All other
transparent media have 𝑛 > 1.
Snell’s Law
There is a relationship between the angle of
incidence, 𝑖, and the angle of refraction, 𝑟.
Normally, we would measure this in the lab.
sin 𝑖
𝑛=
sin 𝑟
e.g. The refractive index of glass is 1.5. What is
the angle of refraction if the light strikes at an
angle of 40°?
Snell’s Law
There is a relationship between the angle of
incidence, 𝑖, and the angle of refraction, 𝑟.
Normally, we would measure this in the lab.
sin 𝑖
𝑛=
sin 𝑟
e.g. The refractive index of glass is 1.5. What is
the angle of refraction if the light strikes at an
angle of 40°?
sin 𝑖
sin 𝑟 =
𝑛
sin 40°
sin 𝑟 =
⟹ 𝑟 = 25.4°
1.5
When light speeds up
The effect is reversed when the light speeds up again.
Virtual Lab: PhET Bending Light
Ignore the secondary reflection.
Only important at night. It is why
a window can act as a mirror.
𝑖
ON button
Vary the angle of incidence
from 0° to 85° and measure
the angle of refraction.
Record the data in a
spreadsheet. Plot a scatter
graph. Don’t try and use a
trendline (won’t work…)
Set to 1.00
𝑟
Set to either water or
glass, then leave alone
What To Hand In
A 1-page lab report. Max font size = 12pt.
• Header with name and date.
• Title
• Background theory
• Screenshot
• Data table and graph (remember column headers and axis labels)
• Conclusion – does the equation work?
Crude Model
of Refraction
The FREQUENCY of the wave does NOT change, therefore,
when it slows, the wavelength shortens.
𝑣 = 𝑓𝜆
More scientific
model
Apparent Depth
Refraction of the light reflected from an object
underwater causes the object to appear to be
shallower than it really is.
This effect is known as apparent depth.
Recall two key points:
• Only the light ray that enters the eye is
relevant
• The brain perceives the light as having
travelled in a straight line.