AndrewMorgan - Workshop 1 (Location): April 2012

15 Questions.

(week 7)

Light and Lenses

1. The speed of light is medium dependant. Which means it changes speed depending on what it is travelling through. Does it get slower or faster travelling through denser mediums. Yes or no?

Yes - The speed of light in a vacuum is 299,792,458 meters per second or 670,615,343 miles per hour. This is represented by the variable c, which stands for the Latin celeritas, which means speed. There is a consensus among physicists that accepting the general theory of relativity, Einstein's prevailing physics theory, entails accepting the speed of light in a vacuum as a constant. Therefore, any experiment proposing that the speed of light in a vacuum changes over time is viewed in the physics community with great suspicion.

It is widely known, however, that the speed of light is variable when the light is not moving through a vacuum. The ratio of the speed of light through a given medium and through a vacuum is called the medium's refractive index or optical density. Some media have such a high refractive index that they can slow light to the speed of a person walking, or even bring it to a temporary standstill.

For example, the speed of light through air is very close to the speed of light in a vacuum. Depending on how dense the transparent media is, it can slow light to a greater or lesser degree. Water and glass can slow light to 3/4 and 2/3 of c, respectively. Different wavelengths of lightalso travel at different speeds through different media. For example, blue light travels a different speed than red light when passing through a prism, causing the two to separate in a process called dispersion.

In reality, the speed of light never actually slows down. It is just delayed as the photons are absorbed and re-emitted by atoms in the intervening space. When a light beam exits a transparent medium into a vacuum, it continues traveling at the same rate as when it originally entered, without any added energy. This shows that slowdown is merely illusory.

At least two media are capable of slowing down light tremendously -- Einstein-Bose condensates, and hot rubidium gas. These have both been used to halt light entirely. This was first achieved on a temporary basis in experiments conducted in 2001.

The above reference was sourced from www.wisegeek.com

2. If light enters glass at an angle other than 90 degrees what happens to it? Explain.

It will become refracted light - Refraction can be defined as the bending of a wave when it enters a new medium. For example, if light is moving through the air, and then it enters some water, it has changed its medium. And you thought medium was just the size of your underwear. Well its not. In wave equations, we often draw a line called the "normal." The “normal” is a line drawn perpendicular to the boundary between the two mediums. If you stood on the boundary between two mediums, with your medium underwear on, and if you were at exactly 90 degrees, would you be normal? Not likely. The normal is just a pretend line that we use as a reference.

Light only refracts when it enters a medium at any angle other than 90. In the diagram below, there is no refraction.

If light hits the boundary at any angle other than 90 degrees, the angle it makes with the normal is called the angle of incidence (angle i). Then, after passing through the boundary, the angle it makes with the normal is called the angle of refraction (angle r). When light travels from a slow medium to a fast medium, it bends away from the normal. When light travels from a fast medium to a slow medium, it bends towards the normal.

The above reference was sourced from http://library.thinkquest.org

3. Light travels in waves. What is the measurement for this?

Wavelengths - and it is measured by calculating the distance between one peak of a wave to the other.

4. What is the degree of the refraction of light dependant on? And which colour is refracted the most?

The degree is dependent on the angle on incidence. Violet has the shortest wavelength, so it is the most refracted colour from the visible spectrum.

5. What is the angle of incidence?

The angle of incidence describes where and at what angle the ray/s of light enter a given medium (i.e. from air into water or glass).

6. What is the refractive index of material?

umm

7. What is the difference between a convergent and divergent lens?

A convergent lens directs light to a focal point, a divergent lens directs light away from a focal point.

Fig. 1.—a, double convex; b, piano-convex; c, concavo-convex or converging meniscus; d, double concave; e, piano-concave; f, divergent meniscus. The first three, a, b, c, which are thicker at the centre than at the margins, are convergent or positive; and the second three, d, e, f, are divergent or negative. All lenses are formed by the union of prisms, and therefore have to a great extent the properties of prisms Fig. 2 will show the way in which the prisms

8. What is the focal length of an optical system?

The focal length of an optical system relates to how powerful the convergent lens' are.

9. What are circles of confusion?

These are simply un-focused rays of light coming through the lens and are more prominent in wide-open apertures. We commonly refer to it as lens blur.

10. What is the angle of view of a lens?

A lens' angle of view represents the distance from its optical centre to the digital camera sensor (or film) when the subject of the photo is in focus. Focal length is measured in millimetres (mm). Put simply, a short focal length means you would need to be close to your subject to get a close up, while a long focal length means you can be far away and still get a close up.

11. What is the covering power of a lens?

Its focal range / area of view.

12. Do lenses alter perspective?

Yes - Perspective distortion is determined by the relative distances at which the image is captured and viewed, and is due to the angle of view of the image (as captured) being either wider or narrower than the angle of view at which the image is viewed, hence the apparent relative distances differing from what is expected.

13. What is geometric distortion?

um something you don't want?!

14. What is the equation to calculate correct viewing distance?

And, PROBLEM to solve: You need to make a vertical portrait using 24x36mm format which will be used on a roadside billboard where the photo will be 5m high and it will be ideally seen from 50m. What lens focal length should you use to maintain ‘correct’ seen perspective? Given: image height = 36mm. Print height = 5000mm (5m). Viewing distance (VD) = 50,000mm (50m). Unknown: magnification (M) = print / image

= 5000 / 36 = 139x focal length (f) = ?

15. For the following lens types, write a description of each  using no more than 10 words for each lens.

Wide angle - large angle of view, edge distortion and close-up exaggeration.

Zoom - ability to focus through a range of view (70-200mm).

Long focus -

Telephoto - fixed focal distance of a long way (400mm).

Retro focus -

Quasi fish eye -

Fish eye - maximum, unrealistic angle of view, distortion and close-up exaggeration.

Catadioptric -

Macro - ability to magnify a small subject for greater detail.

Teleconverter - additional magnification of standard lens (300mm = 420mm)