Đề ôn thi Đề thi vào lớp 10 Trường THPT Chuyên KHXH môn Tiếng Anh - Mã đề KHXH-49

At almost every shop, we see products with black and white stripes. This is a barcode, a visual (35) __________ of data that can be read by a machine. A barcode scanner is a device designed to (36) __________ information encoded in these black and white lines, connecting a physical product to a digital database.

The process starts when the scanner shines a beam of light, typically red, onto the barcode. This light (37) __________ be from a laser or an LED. When the light beam hits the barcode, it is reflected back towards the scanner. The pattern of black bars and white spaces affects how much light is returned.

The key to the system is how different colors reflect light. The white spaces reflect the most light, while the black bars reflect the least. A sensor inside the scanner measures the (38) __________ of the reflected light. The white areas, being more reflective, send back a (39) __________ signal than the black bars.

The sensor converts this varying pattern of light into an electrical signal. (40) __________ is then sent to a part of the scanner called the decoder. The decoder analyzes the signal's waveform to translate it back into the numbers and letters that the barcode represents. (41) __________ does the computer receive the information, but it also instantly looks it up in a database.

This final step is what allows the cashier to see the price and name of the item on their screen. (42) __________, the simple act of scanning a barcode involves a complex but rapid sequence of light, reflection, and digital conversion, making shopping faster and more efficient for everyone.

Look around any cafe or schoolyard in Vietnam, and you will likely see the same sight: teenagers completely absorbed in their smartphones. Whether they are scrolling through TikTok, chatting with friends on Zalo, or studying for an exam, their heads are often tilted down. This common posture has led to a growing health concern that experts call 'text neck'. It's a modern problem for a digital generation, and its effects are more serious than many young people realize, with health professionals in major cities reporting a significant increase in cases.

So, what exactly is 'text neck'? It refers to the stress and pain in the neck and spine caused by looking down at a handheld device for prolonged periods. The human head is surprisingly heavy, weighing about 5 kilograms. When you hold your head upright, your spine can easily support this weight. However, when you tilt your head forward by just 15 degrees, the pressure on your neck increases to about 12 kilograms. A 60-degree tilt, common when looking at a phone in your lap, can feel like 27 kilograms of pressure. The primary symptoms of this condition include chronic neck pain, stiffness in the shoulders, and even frequent headaches.

This issue is particularly worrying for teenagers because their bodies are still developing. Constant pressure on the spine during these formative years can lead to long-term postural problems that are difficult to correct later in life. While adults also suffer from text neck, teenagers are often more vulnerable because of the sheer number of hours they spend on their devices for both entertainment and education. This habit can permanently alter the natural curve of their cervical spine, leading to discomfort and health issues that persist into adulthood.

The negative effects of poor screen posture are not limited to just the neck. Hunching over a phone can also cause significant pain in the upper back and shoulders. Furthermore, this compressed posture can affect other body systems. For instance, it can lead to eye strain from looking at a small screen for too long. In more severe cases, a constantly slumped chest can even reduce lung capacity, making it harder to breathe deeply and efficiently. These connected issues show that how you hold your phone is a whole-body problem.

Fortunately, preventing text neck is possible with awareness and a few simple changes. The most effective strategy is to be mindful of your posture. Try to hold your phone at eye level as much as possible to keep your head up. Take frequent breaks from your screen; a popular method is the 20-20-20 rule, where you look at something 20 feet away for 20 seconds every 20 minutes. Simple neck and shoulder stretches throughout the day can also relieve tension. The key is not to stop using technology, but to use it in a smarter, healthier way.