Magnetism and Electromagnetic waves

Magnetic field A magnet attracts paper clips, nails, and other objects made of iron. Any magnet has two ends, called poles, which is where the magnetic effect is strongest. The pole of a freely suspended magnet that points toward geographic north is called the north pole (N) of the magnet. The other pole points toward the south and is called the south pole (S). A magnet produces a magnetic field (B) that can turn a compass needle. The direction of the magnetic field is defined as the direction that the north pole of a compass needle would point. The magnetic field is measured in tesla (T) or gauss (G), and The magnetic field of a small magnet attached to your refrigerator is about 100 G.

Earth’s magnetic field The north magnetic pole is in the Canadian Arctic, about 900 km from the geographic pole. Magnetism in rocks suggests that the Earth’s poles have not only moved significantly over geologic time, but have also reverses direction 400 times over last 330 million years. The magnetic field of the Earth at its surface is about 0.5 G.

Electric currents produce magnetic fields In 1820, Oersted found that an electric current produces a magnetic field. Direction of the magnetic field produced by currents: right-hand rule. B

Magnetic field and moving charges A moving charged particle would experience a force when passing through a magnetic field.

Mass spectrometer

Electromagnetic induction We have seen a constant current can produce magnetic field. Faraday found that although a constant magnetic field produces no current in a conductor, a changing magnetic field can produce an electric current. Such a current is called an induced current. The phenomenon is called electromagnetic induction.

Lenz’s law The induced current moves in a direction so that the magnetic field created by that current opposes the original change in flux.

Electric generator

Maxwell’s equations Gauss’s law No magnetic charges Faraday’s law Ampere’s law

Electromagnetic waves In vacuum, where there are no charges and currents, the Maxwell’s equations imply the existence of the electromagnetic waves. c is the speed of light.

Light as an electromagnetic wave

Structure of the eye The focusing of the light into image at the retina is produced by the curved surface of the cornea and by the crystalline lens inside the eye. The focusing power of the cornea is fixed. The focus of the crystalline lens, however, is alterable, controlled by the ciliary muscle. 視網膜 角膜 The focusing range of the crystalline lens decreases with age. The near point for a 10-year-old child is about 10 cm, but by the age of 40 the near point shifts to about 22 cm. At age 60, the near point is shifted to about 100 cm.

Retina The retina consists of photoreceptor cells in contact with a complex network of neurons and nerve fibers which are connected to the brain via the optic nerve. There are two types of photoreceptor cells in the retina: cones and rods. The cones are responsible for sharp color vision in daylight. The rods provide vision in dim light.

Aperture and depth of field The iris is the optical aperture of the eye, and its size varies in accordance with the available light. An image is in sharp focus at the retina only for objects at a specific distance from the lens system. Images of objects not at this specific plane are blurred at the retina. A small aperture reduces the diameter of the blurred spot. The range of object distances over which a good image is formed for a given setting of the focus is called the depth of field. Clearly a small aperture increases the depth of field.

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