Introduction
In the International Baccalaureate (IB) Physics syllabus, Option C focuses on the topic of imaging. This study note document will delve into various aspects of imaging, including the principles of optical and electron microscopy, diffraction, and practical applications. The goal is to break down complex ideas into smaller, digestible sections and explain each part clearly, using examples where necessary.
Optical Microscopes
Principles of Optical Microscopy
Optical microscopes use visible light to form an image of a sample. The basic components of an optical microscope include:
- Eyepiece Lens: The lens through which the viewer looks.
- Objective Lenses: Multiple lenses with varying magnification powers.
- Stage: The platform where the slide is placed.
- Light Source: Provides illumination for the sample.
- Coarse and Fine Focus: Adjusts the focus of the image.
Resolution and Magnification
- Resolution: The ability to distinguish between two closely spaced objects. Limited by the wavelength of light used.
- Magnification: The ratio of the apparent size of the object to its actual size.
Note:
The resolution of optical microscopes is limited to around 0.2 micrometres (µm) due to the wavelength of visible light (500-650 nm).
Preparing a Slide
Liquid Specimen
- Add a few drops of the sample to the slide using a pipette.
- Cover with a coverslip, gently pressing to remove air bubbles.
- Wear gloves to avoid contamination.
Solid Specimen
- Cut a small sample of tissue using scissors.
- Peel or cut a thin layer of cells from the tissue sample.
- Treat with chemicals if necessary and stain to make structures visible.
- Place a coverslip on top and press gently to remove air bubbles.
Using an Optical Microscope
- Place the slide on the stage and secure it with stage clips.
- Start with the low-power objective lens to find the sample.
- Use the coarse focusing knob to bring the sample into focus.
- Switch to a higher power lens if needed and use the fine focusing knob to sharpen the image.
Tip:
Always start with the low-power objective lens to prevent damage to the lens or coverslip.
Electron Microscopes
Principles of Electron Microscopy
Electron microscopes use a beam of electrons to form an image, allowing for much higher resolution compared to optical microscopes.
- Transmission Electron Microscopes (TEMs): Use electromagnets to focus a beam of electrons that passes through the specimen. Denser areas absorb more electrons, appearing darker in the image.
- Scanning Electron Microscopes (SEMs): Scan a focused beam of electrons across the surface of the specimen, producing a 3D image.
Advantages and Disadvantages
TEMs
- Advantages: High-resolution images, allows internal structures to be seen.
- Disadvantages: Requires very thin specimens, complex preparation.
SEMs
- Advantages: Produces 3D images, good for surface structures.
- Disadvantages: Lower resolution than TEMs, requires vacuum conditions.
Example:
A TEM can be used to observe the internal structure of a mitochondrion, while an SEM is ideal for viewing the surface topology of a pollen grain.
Diffraction and Imaging
Principles of Diffraction
Diffraction occurs when waves encounter an obstacle or a slit. The extent of diffraction depends on the wavelength of the waves and the size of the obstacle or slit.
- Radio Waves: Wavelengths of 0.1 to 10 m, not easily diffracted by small objects like human hair.
- X-Rays: Wavelengths of $1 \times 10^{-10}$ to $4 \times 10^{-10}$ m, suitable for diffraction by crystalline solids.
Practical Applications
X-Ray Diffraction
Used to determine the atomic structure of crystals. The pattern of diffracted X-rays can be analyzed to reveal the arrangement of atoms.
Note:
X-ray diffraction is crucial in fields like materials science and biology for understanding crystal structures.
Worked Example
Electron Transition in Hydrogen Atom
Question: Which electron transition in the hydrogen atom emits visible light?
- A. $n = 1$ to $n = 2$
- B. $n = 2$ to $n = 3$
- C. $n = 2$ to $n = 1$
- D. $n = 3$ to $n = 2$
Answer: Option D is correct. Emission in the visible region occurs for an electron jumping from any higher energy level to $n = 2$.
Conclusion
Understanding the principles of optical and electron microscopy, as well as the concept of diffraction, is crucial for mastering the topic of imaging in the IB Physics syllabus. By breaking down these complex ideas and providing practical examples, students can gain a deeper appreciation of the techniques used to visualize the microscopic world.
Common Mistake:
A common mistake is to confuse resolution with magnification. High magnification does not necessarily mean high resolution.