【Science】1st Year: Master the World of Light and Sound!
Hello everyone! In this chapter, we will learn about "light" and "sound," which surround us every day. Let’s use the power of science to solve everyday mysteries like, "Why can I see my reflection in a mirror?" or "Why is there a delay between seeing a flash of lightning and hearing the thunder?"
If you feel like science isn't your strongest subject, don't worry! If you grasp these points one by one, you’ll definitely be able to master them!
1. Properties of Light (Reflection and Refraction)
Light usually travels in a straight line through the air. This is called the rectilinear propagation of light. However, when it hits something or enters a different substance, its movement changes.
① Reflection of Light
When light hits an object like a mirror and bounces off, this is called reflection.
The rule you absolutely must remember here is the "Law of Reflection."
Key Points:
・Angle of Incidence: The angle at which light hits a surface.
・Angle of Reflection: The angle at which the light bounces off.
These two are always equal (Angle of Incidence = Angle of Reflection).
Common Mistake:
When measuring these angles, be sure to measure from the "normal line" (a line perpendicular to the surface), not from the surface of the mirror itself. If you get this wrong, your calculations will be off, so be careful!
② Refraction of Light
When light moves into a different type of substance—such as moving from "air to water" or "air to glass"—it bends at the boundary. This is called refraction.
・When entering water/glass from air: Angle of Incidence > Angle of Refraction (The light bends inward).
・When exiting water/glass into air: Angle of Incidence < Angle of Refraction (The light spreads outward).
Example: A straw in a glass of water looks bent because of this refraction.
③ Total Internal Reflection
When light travels from water or glass into the air, if you keep increasing the angle of incidence, there comes a point where the light can no longer pass through and instead reflects entirely back into the original substance. This phenomenon is called total internal reflection.
Technologies like fiber optics use this property to transmit information over long distances.
★ Summary so far:
For reflection, remember "Angle of Incidence = Angle of Reflection." For refraction, remember "it bends inward when entering from air!"
2. The Function of a Convex Lens
A lens that is thicker in the middle, like a magnifying glass, is called a convex lens. In exams, the most important points are understanding "how light converges and what kind of image is formed."
① Important Terms
・Focus (Focal Point): The point where light rays passing through the lens converge.
・Focal Length: The distance from the center of the lens to the focus.
② How Images are Formed (Two Patterns)
The appearance of the image changes depending on the distance between the object and the lens.
1. Real Image:
An image formed when the object is placed further away than the focal point. It can be projected onto a screen, and the image will be upside down and reversed left-to-right.
2. Virtual Image:
An image formed when the object is placed closer than the focal point. This is the image you see when looking through the lens; it is upright and larger than the actual object (like using a magnifying glass).
Tip:
Try remembering it this way: "Real is flipped (upside down), Virtual is as-is (upright)." This will help you avoid mixing up the orientations!
3. Properties of Sound
Next, let's look at "sound." The true nature of sound is "vibration."
① How Sound Travels
The object producing the sound is called the sound source. Sound travels as a vibration through media like air.
Important: Sound cannot travel in a vacuum (a place with no air). If you shouted in outer space, no one would be able to hear you.
② Speed of Sound
Sound travels through the air at approximately 340 m/s. This is much slower than the speed of light (300,000 km/s).
The reason there is a gap between seeing the flash of lightning and hearing the thunder is due to this difference in speed.
Calculation Formula: \( \text{Speed of sound} [m/s] = \frac{\text{Distance traveled} [m]}{\text{Time taken} [s]} \)
③ Loudness and Pitch of Sound
How we hear sound is determined by "amplitude" and "frequency." Imagine a monochord (an instrument with a stretched string).
・Loudness: If you pluck the string harder, the vibration (amplitude) becomes larger, creating a louder sound.
・Pitch: If you tighten the string, shorten it, or use a thinner one, the number of vibrations per second (frequency) increases, creating a higher sound.
Point:
The unit for frequency is Hz (Hertz). The higher the number of vibrations per second, the higher the pitch.
★ Summary of Sound:
"Amplitude = Loudness," "Frequency = Pitch." For oscilloscope waveform problems, remember that the vertical width of the wave represents "loudness," and how compressed the wave looks (how many peaks/valleys) represents "pitch!"
Closing: Study Advice
The best shortcut for the "Light and Sound" unit is to draw diagrams. Especially for the path of light through a convex lens or the angles of refraction, try copying the diagrams from your textbook into your notebook yourself.
It might feel difficult at first, but the rules are simple. If you proceed one step at a time, asking "Why does it happen this way?", it will start to feel as fun as solving a puzzle!
I’m rooting for you!