Physical and geometric optics

Learning object 8 - Zeanna Janmohamed

Figure 1.0 optics diagram: http://4.bp.blogspot.com/-u-JvAmYr8dQ/VL6O4xnORXI/AAAAAAAAAl8/jaHgHuld-w8/s1600/convex_sign_convention.png

Geometric ray optics

• A branch of physics that uses reglcyion and refraction to understan how images are formed – Upsides? Its accurate as long as the wavelength of

the light is smaller than the dimensions of the object

– What happens when we can no longer use ray optics? • New field is developed!

Wave physical optics

• Used when conisdering the interference between waves travelling in different paths

• Can be used for any electromagetic radiation (not only light, but because light has large wavelengths it is easier to use)

Figure 1.1 electromagnetic spectrum http://www.yorku.ca/eye/spectrum.gif

Check and reflect• Question 1. Is it appropriate to

use x-rays for visualizing your intermediate phlanges (a bone in your finger)? How can you explain why by using gemetric optics or physical optics.

• Question 2. Is it appropriate to useinfared to shine on a red blood cell? How can you explain why by using gemetric optics or physical optics.

Figure 1.3 intermediate phalanges http://upload.wikimedia.org/wikipedia/commons/1/11/Intermediate_phalanges_of_the_hand_(left_hand)_02_dorsal_view.png

Figure 1.4 red blood cellhttp://news.rice.edu/wp-content/uploads/2012/09/0914_BLOOD_lg.jpg

Check and reflect answers

1. Yes it is appropriate. We can surmize this because the size of your intermediate phalange is between 2-6cm, and x rays are considerably smaller at 0.01 o 10nm, and therefore gemetric optics can be used to evalute this problem.

2. No it is not appropriate, although it can be done. We can surmize this because the size of your blood cells are around 8 micrometers in width. Comparivley, infa rays are between 0.7 to 300 micrometers, and are similar, if not larger to the red blood cells. Therefore you must use physical optics to analyse the situation

Geometrical optics = thin lenses • A lense is a refracting device = redistribues the

energy that is propagated by the EMR • Converging or convex lenses are thicker at

their midpoint and taper off at the end• Diverging or concave lenses are thicker at their

ends and thin down at the middle

Figure 1.6 converging and diverging lenses http://www.physicsclassroom.com/Class/refrn/u14l5a1.gif

Geometrical optics = thin lenses cont.

• Lenses that have two surfaces are known as simple

• If the thickness is negligible compared to the overall path of light the lenses are called thin

• the formula for the lens is now: – N1 = index of refraction, R2 = radius of curvature

of left surface, r2 = radius of curvature of right surface

Figure 1.7 lens formathttp://www.sparknotes.com/physics/optics/geom/section3.rhtml

Geometric optics= mirrors

• Concave mirrors = reflect the incoming waves to a focal point in front of the mirror

• Convex mirrors = reflect incomping waves outwards so that the image appears behind the mirror

Figure 1.8 concave and convex mirrors http://buphy.bu.edu/~duffy/PY106/22c.GIF

Further analysis

• Follow this link and see how the images are formed by converging lesnes. Observe how changing the radius and the refractive index of the lenses affects the image that is formed (what do you notice? Do they follow the patterns explained by converging or diverging lenses )

• http://phet.colorado.edu/en/simulation/geometric-optics