Reflection of Light.
When a ball is thrown against a wall, what is the path through which it goes called?
The line of incidence.
When the ball bounds back, what is its path called?
The line of reflection.
What is the angle at which a ball strikes the wall called?
The angle of incidence.
What is the angle at which the ball rebounds called?
The angle of reflection.
When a ray of light falls upon a looking-glass, what is the path through which it goes called?
The line of incidence.
When the ray is reflected, what is its path called?
The line of reflection.
The angle of incidence is the angle between a perpendicular and the line of incidence; and the angle of reflection is the angle between the perpendicular and the line of reflection. The two angles are always equal to each other, as is represented in [Fig. 8].
Fig. 8.
When a ray of light strikes a looking-glass obliquely or slantingly, what is done with it?
The ray is reflected as obliquely from the glass as it strikes or falls upon it.
Why is the reflection of the sun in a pond of water seen near the edge at noon?
Because the angle of reflection is equal to the angle of incidence; and the observer must be nearly over the place where the rays strike, so that, when reflected, they may meet his eye.
Where is the reflection seen in the morning or in the evening?
It is seen nearer the middle of the pond; because the sun’s rays then fell more obliquely on the pond, and are reflected more obliquely to the eye.
When an opaque object is placed between a candle and a wall, why will there be a shadow?
Because the opaque object prevents the rays of light from falling on the wall.
Why will the shadow be larger, the nearer the object is to the candle?
Because the rays of light diverge, in every direction from the candle, in straight lines, and the nearer the object, the more rays will it intercept, as is represented in [Fig. 9].
Fig. 9.
In [Fig. 9], let L represent the flame of a candle. When the object is placed at A, the shadow on the wall will extend from C to D; but when the object is moved nearer, as at B, the shadow will extend from E to F.
Why does an object seem to be smaller, the farther it is from us?
Because the angle at which the light from it strikes the eye, is less when at a distance, than when near to the eye.
Fig. 10.
In [Fig. 10], the diameter of the near circle is from A to B; but the diameter of the more distant circle, although just as large, seems only to be from C to D.
Why does an object grow more dim the farther it is from us?
Because the rays of light from it spread out in every direction; and fewer rays enter the eye when it is at a distance than when it is near by.
What is the use of the telescope?
It is used to view distant objects.
Why can we see a distant object more distinctly with a telescope, than without one?
Because the telescope enlarges the image of the object, and it also collects more light from that object than is collected by the unaided eye; thus, making the image distinctly visible.
What class of objects do we view with a telescope?
We view the heavenly bodies; the sun, moon, and stars.
What is a spy-glass?
It is a glass mostly used to view distant objects on the earth.
Is the spy-glass like the telescope?
The spy-glass is much like the telescope, only it is smaller.
When looking through a spy-glass, how do objects appear?
They seem to be brought nearer to us, and they appear larger.
By whom are spy-glasses much used?
Spy-glasses are much used by persons on board of vessels, while at sea.
What instruments have we for magnifying small objects?
We have magnifying glasses and microscopes.
What are magnifying glasses?
They are single glasses or lenses, used for magnifying small objects.
By whom are magnifying glasses used?
They are used by jewellers, watchmakers, engravers, and others who examine small objects.
What are microscopes?
They are instruments used for examining very small objects.
How does a drop of rain-water or vinegar appear under the microscope?
It appears to be full of strange-looking creatures, which are always in motion.
How do the smallest insects appear?
They appear to be as perfectly formed as the larger ones which are seen with the naked eye.
What may be seen with a microscope?
Thousands of things too small to be seen by us unless they are magnified.
What kind of spectacles do near-sighted people use?
Near-sighted people use glasses, thickest at the edge and thinnest at the centre.
What kind of spectacles do we use as we grow old?
As we grow old we use glasses, thinnest at the edge and thickest at the centre.
Of what use are spectacles?
When the vision is defective, spectacles enable us to see objects more clearly.
How should spectacles be made?
They should be so made as to render objects distinct, but neither to magnify nor diminish them.
As people grow old, how do their eyes change?
Their eyes change so that they see objects at a distance better than those which are nearer, hence, in reading they hold a book farther from the eye.