Light is both waves and particles. By wave we mean light that has wavelength and interference properties. By particle nature we mean light has momentum and energy just like ordinary particles in motion.

All light waves travel in vacuum with same velocity c = 300,000 km/sec.

The product of wavelength and frequency is velocity. c = l f.

Different colors are different wavelengths or frequency.

Energy in a wave is proportional to the frequency.

Most of the light waves are not visible to the eye. Examples in increasing order of wavelength are gamma rays, X-rays, ultra violet, visible, infrared, microwaves and radio waves.

 Shadows: absence of light rays caused by an opaque (non-transparent) object.

Shadows of sunlight caused by the Moon are eclipses. Solar eclipses occur about once every 1.5 years somewhere on the surface of the Earth.

 Images: real images are formed when rays of light focused on a surface (such as photographic film) forming an image. Virtual images are formed when the eye focuses the light rays and an image are formed in the eye.

 Reflection of light: A change of direction of a light wave by a surface. The angle of incidence measured with respect to the normal (perpendicular to the surface) is equal to angle of reflection.

 A mirror is a good flat reflector surface. When we look in a mirror the image we observe is actually in our eye.

 A reflecting surface that is not flat can be a concave mirror or a convex mirror.

A concave mirror focuses parallel rays of light to a point called focus. Focal length is the distance between the center of the mirror and the focus point.

 When an object is at a distance from a concave mirror that is less than the focal length the image is magnified. Remember the eye forms the image.

 Why is the sky blue? Blue light rays scatter more than red light. When the sun rays travel through a lot of air, such as during sunrise or sunset, most of the blue light is scattered out and what remains is red. When we look at the sky away from the Sun, what we observe are the scattered light, which is predominantly blue.

Whenever light travels in a transparent medium such as water or glass, the velocity is reduced. The ratio of velocity of light in vacuum (c) and velocity in the medium is the index of refraction (n). The frequency remains the same, but the wavelength is also reduced by n.

 Whenever light enters a medium the angle changes more towards the normal. Snell's Law gives the relationship between the angle of incidence and angle of refraction. A pencil partly in water appears bent at the water surface. This is due to the bending of light rays at the surface. The same principle also makes images appear closer to the surface then the objects are really located.

 The index of refraction of the atmosphere is close to one, but the amount above one is the cause of atmospheric refraction that makes the Sun appear higher above the horizon than its real location.

 Index of refraction is wavelength dependent. That is different wavelength waves bend by different amounts. This is the reason a prism breaks sunlight into its colors. The same is also true with raindrops that form rainbows.

 Internal reflection: whenever light leaves a medium, the angle the ray emerges is greater than the incidence angle. If the angle of emergence is greater than 90 degrees, it is totally internal reflection.

 Lenses.

A converging lens focuses parallel light to a point (focus). The distance between the focus and lens is the focal length. Ray tracing can locate the image formed by a lens. A lens that is thicker at the center is called converging lens. A lens that is thicker at the edges is called diverging lens. Magnifying lens: when an object is at a distance that is inside the focus, the image is virtual and magnified. Camera: the lens forms a real image that is captured on film or electronic media.

 Eye: the eye is a lens that forms a real image on the retina. If the image is not focused on the retina, eyeglasses correct this deficiency. The power of an eyeglass is the inverse of the focal length in meters and is measured in units of diopters. A converging lens has +ve diopters and a diverging lens has -ve diopters.

 Double lens systems are used to build special equipments, such as telescopes, microscopes, binoculars, etc. Refracting telescopes use two converging lenses, while reflecting telescopes have one concave mirror and one converging lens.

The wave properties of light can be observed in interference and diffraction.

Interference: the addition of two waves that can add or subtract and produce interference patterns. Diffraction: spread of waves while passing through a small hole. Another wave property is polarization, where the transverse vibrations are all in a single plane.

 Interference and diffraction can be computed exactly and depend on the wavelength. In general the longer the wavelength, the bigger the diffraction effects. The limitations of a microscope or a telescope are due to diffraction. The shorter the wavelength, the smaller these effects. Interference can also occur when we have two close-by surfaces.

 Diffraction grating has many close-by slits, which magnifies the interference effects. It sometimes is used to separate the wavelengths similar to a prism.

 Resolution: how well two close-by objects can be separated. The resolving power of a telescope is limited by diffraction and the larger the objective lens, the better is the resolution.