The Speed of Light: Galileo Galileo (Italian) in early 1600s attempted to determine light’s speed by opening a lantern which would be seen by a person miles away who would then open his lantern that then would be seen by Galileo in turn. Galileo would record the time from opening his lantern to when he saw his assistant’s light. This experiment failed. If Galileo’s assistant were 10 km away, it really only took 6.7 x s for light to travel this distance, much less than the human reaction time to record this.

Olaus Romer’s Measurement of Light’s Speed In the late 1600s Romer (Danish) used astronomers’ measurements of the time difference for an eclipse of one of Jupiter’s moons. A difference of 1320 s existed for the eclipse when viewed from earth at it closest and farthest points in orbit to Jupiter. The diameter of earth’s orbit was known to be 3.0 x m, so it took light 1320 s to travel 3.0 x m which comes out to a speed of 2.3 x 10 8 m/s.

Albert Michelson in 1905 In 1905 Albert Michelson (American) made a very accurate measurement of light’s speed. He used a rotating, 8-sided mirror which sent light a distance of about 35 km away. The mirror had to rotate at times/min ( rot./s) for the light to make the journey away from the mirror and back to the mirror as it made 1/8 (.125) of a turn. To make 1/8 of a turn took x s, so light travelled m in x s which is a speed of x 10 8 m/s. The accepted value today is x 10 8 m/s

Images in a Greek Temple The Greeks noticed that if a dark room had a hole, an image of outside objects formed on the wall opposite the hole.

Artists and the Camera Obscura (Dark Room) During the Renaissance, artists used the camera obscura, a dark box with a hole in one end and a frosted plate glass opposite this, to discover the laws of perspective enabling them to draw and paint very realistically.

The Pinhole Camera The pinhole channels light rays to form an inverted image opposite the pinhole.

Placing a Lens in Front of the Hole A lens collects more light and makes a brighter image. Lenses were used after the Renaissance.

The Magnification Equation of the Pinhole Camera Because of similar triangles, the ratio of height of the image to the height of the object is equal to the ratio of the distance of the image (from the pinhole) to the distance of the object (from the pinhole). Hi/Ho = Di/Do

Early Cameras : Bulky and Messy Early cameras were heavy, took time to set up. Early film needed to be made on the spot and developed on the spot in a dark room.

George Eastman and the Kodak Brownie Camera Eastman made photography accessible for the average person.

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