Congratulations! We have just discovered the earliest model of photo camera, Camera obscura.

1. Congratulations!We have just discovered the earliest model of photo camera, Camera obscura. What are the drawbacks of the camera obscura?

2. What kind of light is emitted by regular (not self-luminous) objects?

3. How do we see objects? Our vision involves formation of images of different objects on the retina of our eye.

4. Reflection of Light • A ray of light, the incident ray, travels in a medium • When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium

5. The law of reflection A single ray of light is incident upon a reflecting surface at an angle q1 with the normal to the surface.It is reflected at an angle q1’with the normal, such that q1’ = q1.The angle of reflection is always equal to the angle of incidence.

6. The law of reflection If the surface is flat and smooth, and the rays in the incident beam of light are parallel to each other (plane wave), they will all be reflected at the same angle.This is the case of specular reflection. If the surface is rough, the law of reflection will still work for every ray and every small element of surface. The law of reflection will not work for the beam, however, because of random orientation of different surface elements. Different rays constituting the beam will be reflected in random directions, which is diffuse reflection.

7. Diffuse reflection:a parallel beam is reflected as bunch of diverging rays Specular reflection:an incident parallel beam is reflected as a parallel beam

8. What type of reflection is more useful? Well, it depends, what you actually want to get…

9. Which part of the figure below shows specular reflection of light from the roadway? With diffuse reflection you see the road;With specular reflection you see the lights of the car reflected off the road.

10. Mirrors are designed to give nice specular reflection. Therefore the mirror surface is invisible by itself.There are no point sources of light on mirror surface. Therefore, looking into a mirror you see reflections of objects, not the mirror itself.

11. Refraction of Light • When a ray of light traveling through a transparent medium encounters a boundary leading into another transparent medium, part of the ray is reflected and part of the ray enters the second medium • The ray that enters the second medium is bent at the boundary • This bending of the ray is called refraction

12. Refraction of Light. • The incident ray, the reflected ray, the refracted ray, and the normal all lie in the same plane • The angle of refraction, q2, depends on the properties of the two media and speeds of light in them, v1,2 = c/n1,2

13. Following (tracing) the Reflected and Refracted Rays • Ray  is the incident ray • Ray  is the reflected ray • Ray  is refracted into the lucite • Ray  is internally reflected in the lucite • Ray  is refracted as it enters the air from the lucite

14. The angle of refraction depends upon the speeds of light in the two materials and the angle of incidence • The path of the light through the refracting surface is reversible • Refraction occurs because the speeds of light, v1 and v2, are different in the two media • The index of refraction, n, of a medium can be defined

15. Snell’s law of refraction is written in a form symmetric to the incident and refracted beams: • In terms of indices of refraction the law of refraction becomes • Sine of angle of incidence (refraction) is inversely proportional to the index of refraction of the medium

16. Media with high n, like diamond, are sometimes called “optically dense”. • For a vacuum, n = 1 • For other media, n > 1 • n is a unitless ratio, n = c/v

17. When light refracts into a material, where the index of refraction is higher, the angle of refraction is less than the angle of incidence • The ray bends toward the normal • When light refracts into a material, where the index of refraction is lower, the angle of refraction is greater than the angle of incidence • The ray bends away from the normal

18. A material has an index of refraction that increases continuously from top to bottom. Of the three paths shown in the figure below, which path will a light ray follow as it passes through the material? (b). When light goes from one material into one having a higher index of refraction, it refracts toward the normal line of the boundary between the two materials. If, as the light travels through the new material, the index of refraction continues to increase, the light ray will refract more and more toward the normal line.

19. q1 45° n1= 1 q2 5° n2= ? • The angle q1is too small to be real !