As atoms absorb energy, electrons jump out to a higher energy level.
Electrons release light when falling down to the lower energy level.
Photons - bundles/packets of energy released when the electrons fall.
Light as stream of photons
Color of Light
1. Transparent Objects
Light transmitted because of no scattering
Color transmitted is color you see. All other colors are absorbed.
2. Translucent
Light is scattered and transmitted some.
3. Opaque
Light is either reflected or absorbed. Color of opaque objects is color it reflects.
White light is the presence of ALL the colors of the visible spectrum.
Black objects absorb ALL the colors and no light is reflected back.
Light and its Sources
Luminous object
Luminous objects have their own light example : sun, firefly, light worm
Hence, non luminous objects reflect light example : moon, hand
Incandescent light
Light produced by heating an object until it glows.
Fluorescent Light
Light produced by electron bombardment of gas molecules
Phosphors absorb photons that are created when mercury gas gets zapped with electrons. The phosphors glow & produce light.
Neon light
Neon inside glass tubes makes red light.
Other gases make other colors.
Reflection of Light
Reflection – Bouncing back of light waves at the same angle (angle of incident = angle of reflection or α = β) and same plane. The green dashed line is showing the normal line to help us identify the angle of incident (the angle between the incident ray and the normal line) and the angle of reflection (the angle between the reflection ray and the normal line), the normal line should be perpendicular to the plane.
Kind of reflection :
1. Regular reflection – mirrors smooth surfaces scatter light very little. Images are clear & exact. 2. Diffuse reflection – reflected light is scattered due to an irregular surface.
Exercise :
1. Draw the reflection of the light when a light enter at the angle of 20 to the plane
2. Draw the reflection of the light when a light enter at the angle of 45 to the normal line
Term of Image / Shadow
Real Image - Can be projected onto a screen because light actually passes through the point where the image appears , always inverted
Virtual Image - “Not Real” because it cannot be projected, image only seems to be there
Upright – Image' position is the same as the object
Inverted – Image is upside down.
Laterally inverted – Image is right side left
Enlarged – Image is larger than actual object.
Reduced / Diminished – Image is smaller than object.
Converge / Converging – Bring the light to a point
Diverge / Diverging – Diverge the light to many points.
Principal Axis – Base line through the center of a mirror or lens
Focal Point – Point where reflected or refracted rays meet & image is formed
Focal Length – Distance between center of mirror/lens and focal point
Centre Point – Centre point of a mirror or lens
Plane Mirror – Perfectly flat
Image is virtual, laterally inverted
An angled mirror will create number of shadows as (360 / α) – 1 where α is the angle. For example two mirrors when placed to create an angle of 90 will give us 3 shadows
Convex Mirror (Diverging Mirror)
- Curves outward, virtual focal point.
Image is virtual, laterally inverted, reduced, upright, closer than it appears
Use: Rear view mirrors, store security
Ray #1: Light ray comes from top of object will travel parallel to principal axis is reflected as if it come from focal point.
Ray #2 : Light ray comes from top of object to the center of mirror will be reflected back.
Ray #3 : Light ray comes to the focal point will be reflected to the principal axis
Exercise :
Find the image and its characteristics by drawing and calculating in front of convex mirror with the centre point of 5 cm when the object is placed
(1) 8 cm in front of the mirror
(2) 3 cm in front of the mirror
(3) 1 cm in front of the mirror
Concave Mirror (Converging Mirror)
Curves inward, real focal point.
Ray #1: Light ray comes from top of object will travel parallel to principal axis will be reflected to focal point.
Ray #2 : Light ray comes from top of object travels through center of mirror will be reflected back.
Ray #3 : Light ray comes to the focal point will be reflected parallel to the principal axis
Exercise :
Find the image and its characteristics by drawing and calculating in front of concave mirror with the centre point of 5 cm when the object is placed
(1) 8 cm in front of the mirror
(2) at the centre point of the mirror
(3) 3 cm in front of the mirror
(4) at the focal point of the mirror
(5) 1 cm in front of the mirror
If your drawing is right, you will find the characteristics as (1) real, inverted and reduced (2) real, inverted and same size (3) real, inverted and magnified (4) infinity image (5) virtual, upright and magnified
Therefore we can conclude, special for concave mirror, we can categorized 5 zone of object and image which simplify us to know the characteristics of the image:
1. Object in zone 1 (from the mirror to the focal point) will create image in zone 4 (at the back of the mirror) which means virtual, magnified and upright
2. Object in zone 2 (from the focal point to the centre point) will create image in zone 3 (further than its centre point) which means real, magnified and upside down
3. Object in zone 3 (further than its centre point) will create image in zone 2 (from the focal point to the centre point ) which means real, reduced and upside down
4. Object in the focal point will create infinity image
5. Object in the centre point will create the image at the same place which means real, same size and upside down.
Now compare your drawing (the place of your image) with the formula:
1/f = 1/s + 1/s’ where f is focal length and s is distance of the object and s’ is distance of the image
M = h’/h = s’/s where M is magnification and h is height of the object and h' is the height of te image. When magnified, the value of M is more than 1. Then if it is reduced the value is less than 1. Last, if the value of M is 1, it means that the image has a same size as its object.
Both should show the same place, or it means that there is something wrong with your drawing or calculation.
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