Transcript solar.gmu.edu

Properties of Light
Lab 5
Basic Properties of Rays of Light
• Light rays travel in a straight line
• Angle of the incident ray of light is always equal
to the angle of the reflected ray of light
• Light rays bend or refract when they pass from
one medium to another
• Refraction is caused by the change in speed
experienced by a wave when it changes medium
• Light can either slow down while crossing the
boundary and refract towards the normal or
speed up while crossing the boundary and
refract away from the normal
Refraction of Light
• Regardless of what the two media are,
there is a definite relationship between the
sines of the angles of incidence and
refraction – Snell’s Law
• Applet of Snell’s Law –
http://lectureonline.cl.msu.edu/~mmp/kap2
5/Snell/app.htm
Concave and Convex Lenses
Focal Point
• The focal point of a concave lens is the
point where light rays parallel to the axis
seem to diverge from after passing
through the lens
• The distance from the lens to this point is
called the focal length of the lens
Convex and Concave Lenses
• http://microscopy.fsu.edu/primer/java/com
ponents/perfectlens/
• http://www.phys.hawaii.edu/~teb/optics/jav
a/clens/
• http://www.mtholyoke.edu/~mpeterso/clas
ses/phys301/geomopti/lenses.html
Parallax
• Parallax is the apparent shift of an object against
a background due to a change in observer
position
• By observing parallax, measuring angles, and
using geometry, one can determine the distance
to various objects
• Distance measurement by parallax is a special
case of the principle of triangulation, where one
can solve for all the sides and angles in a
network of triangles if, in addition to all the
angles in the network, the length of only one
side has been measured
Parallax triangulation
• In parallax, the triangle is extremely long and
narrow, and by measuring both its shortest side
and the small top angle α, the long sides can be
determined and are assumed to be
approximately equal to each other
How it works…..
• b covers an angle α
2π r covers an angle 360°
• so 2π r = (360°/ α b)
• r = (360°/2 π α) b
• If we know b, we can deduce r
• For example, if we know that α = 5.73°,
then 2 π α = 36° and r = 10 b
Use it to estimate distances …
• Stretch your arm forward and extend your
thumb, thumb facing your eyes
• Close one eye and move your thumb so that,
looking with your open eye , you see your
thumbnail covering the landmark A
• Then open the eye you had closed and close the
one with which you looked before, without
moving your thumb
• It will now appear that your thumb has moved
and is no longer in front of landmark, but in front
of some other point at the same distance
• The distance to the landmark is 10 times
the distance between the 2 places your
thumb appeared to be
• The angle between the lines from the eyes
to the outstretched thumb is about 6°, for
which the ratio is 1:10
• That angle is the parallax of your thumb,
viewed from your eyes
Stellar parallax
• http://instruct1.cit.cornell.edu/courses/astr
o101/java/parallax/parallax.html