Light
Reflection of light
What is reflection?
the bouncing of light off a surface
when white light is incident on a red apple, with the exception of red light, all light is absorbed by the apple while red light is reflected into our eyes
the set of primary colours of light is red, green and blue
red and green = yellow
red and blue = magenta
green and blue = cyan
Ray diagrams
incident ray - the light ray hitting the reflecting surface
reflected ray - the light ray travelling away from the reflecting surface after bouncing off the surface
normal - the line perpendicular to the surface at the point of incidence
angle of incidence (i) - the angle between the incident ray and the normal
angle of reflection (r) - the angle between the reflected ray and normal
Mirrors
Characters of images formed by plane mirrors
the image is of the same size as the object
the image is upright
the perpendicular object distance is equal to the perpendicular image distnace from the plane mirror
the image is laterally inverted
the image is virtual
Laws of reflection
incident ray, reflected ray and the normal at the point of incidence all lie in the same plane
angle of incidence (i), is equal to the angle of reflection (r)
Applications of plane mirrors
periscopes
ammeter/voltmeter
Concave mirrors
mirrors with a curved surface that bends inwards
the images formed are virtual, upright and magnified
used when a magnified image is needed
Convex mirrors
mirrors with a curved surface that bends outwards
the images formed are virtual, upright, and diminished
provides a wide field of vision and are commonly used when a large area needs to be seen
Regular reflection
when light is incident on a surface, the light ray obeys the laws of reflection
as parallel beam of light rays is incident on a smooth surface, the angles of incidence at all the points are the same - all angles of reflection are the same as the light beam is evenly reflected
the observer will be able to see the image clearly, and the reflecting surface appears brightly lit
Diffused reflection
when light is incident on a rough surface, the light ray obeys the laws of reflection
each point of incidence has a different normal from that at other points, the angle of incidence made by the incident ray at each point of incidence may be different
the angles of reflection made by the reflected rays will be different from each other - the reflected rays will no longer form a beam of parallel rays
the observer will not be able to see the reflected images clearly and the reflecting surface will not be as bright
Refraction and dispersion of light
What is refraction of light?
when light travels across different media, its speed changes
Refractive index
tells us how much a material is able to slow down the speed of light as it travels through the material
refractive index (n) is defined as the ratio of the speed of light in vacuum to the speed of light in the medium ( n = c/v) where c is the speed of light in the vacuum (3.00 x 10(8) m/s), v is the speed of light in the medium
Conditions of refraction of light
the light ray passes from one medium to another of different optical density
the angle of incidence must be more than 0 degrees
Properties of refraction of light
light bends towards the normal if it travels into another medium of higher refractive index
light bends away from the normal if it travels into another medium of lower refractive index
What is dispersion of light?
when white light passes through a different medium, each of the 7 colours of light slows down to a different extent
the light rays for each colour refracts by different angles, resulting in the spreading out of the seven colours
Application of light
infrared - thermal imaging cameras to screen people for fever - overexposure can damage our eyes
visible light - allows plants to photosynthesise - cause light pollution
ultraviolet - to disinfect medical equiptment and food - overexposure can harm our eyes and cause skin cancer